How do i scan a legal size document?

it seems that on your model you can only use legal size paper on tray 1 & 2, then you have to set the paper size.
here is a link to instructions for your model.
http://download.brother.com/welcome/doc002258/mfc9840_uk_usr_b.pdf

you might confirued wrong paper setting on print preview of scan document or try to change the size of the paper to A4 or letter

Make sure to set the correct paper size when scanning from the ADF. If you're scanning a legal size document, set the paper size to legal.

If it doesn't work, try to reset the printer back to its factory settings:

1 From the home screen, touch Setup (wrench icon), and then touch Use Factory Defaults.
2 Touch Yes.
3 When prompted to set the initial settings of your printer, make your selections using the touch screen.

I had exact same issue, could not scan in any size larger than Letter; changed Scan Default settings for paper size but made no difference. Based on dmf1984's suggestion I did same: went to Start -> Dell -> Dell Printers -> Dell Laser 1815dn -> Network Scan and changed Scan Paper size to Legal hit apply then tried scan again... problem solved

MFP and scanner settings can be changed several ways: through the MFP control panel, the software application in use,
the print driver, MarkVision™ Professional, or the Web pages. Settings changed from the application or print driver apply
only to the job being sent to the MFP.
Changes made to print, fax, copy, and e-mail settings from an application override changes made from the control panel.
If a setting cannot be changed from the application, use the MFP control panel, MarkVision Professional, or the Web pages.
Changing a setting from the MFP control panel, MarkVision Professional, or the Web pages makes that setting the user
default.
Menus (A key is shown on the button.) Touch to access the menus. These menus are only
available when the MFP is in the Ready state.
The Menus button is on a gray bar called the navigation
bar. The bar contains other buttons described as follows.
Understanding the copy screens and options
Copy From: Opens a screen where you can enter the size of the documents you are going to copy.
• Touching any of the paper size icons on the follow-on screen will make that your selection and return you to the
Copy Options Screen.
• When Original Size is set to Mixed Sizes, you can scan different size original documents (letter and legal size
pages). They will be scaled automatically to fit the selected output media size.
Copy To: Opens a screen where you can enter the size and type of paper on which your copies will be printed.
• Touching any of the paper size icons on the follow-on screen will make that your selection and return you to the
Copy Options Screen.
• If the size settings for Copy From and Copy To are different, the MFP will automatically adjust the Scale setting to
accommodate the difference.
• If you have a special type or size of paper that you'd like to copy onto that is not normally in the MFP paper trays,
you can select MP Feeder, and manually send the type of paper you want through the MP feeder.
• When using Auto Match, the MFP will match the size of the original document as specified in the Copy From box.
If a matching paper size is not loaded in one of the paper trays, the MFP will scale the copy to fit loaded media.
Scale: Creates an image from your copy that is proportionally scaled anywhere between 25% and 400%. Scale can also be
set for you automatically.
• When you want to copy from one size of paper to another, such as legal size to letter size, setting your Copy From
and Copy To paper sizes will automatically change the scale to keep all the original document information on your
copy.
• Touching the arrow to the left lowers the value by 1%; touching the arrow to the right increases the value by 1%.
• Holding your finger on an arrow makes a continuous increment.
• Holding your finger on an arrow for two seconds causes the pace of the change to accelerate.
************************Darkness: Adjusts how light or dark your copies will turn out in relation to the original document.****************** here is the link to the full pdf manual for your unit: http://www.lexmark.com/publications/pdfs/2007/x644e/en/ug.pdf#xml=http://support.lexmark.com/lexmark/index?page=answeropen&segment=SUPPORT&userlocale=EN_US&locale=en&type=open&searchid=1270220491739&answerid=16777216&iqaction=6&url=http%3A%2F%2Fwww.lexmark.com%2Fpublications%2Fpdfs%2F2007%2Fx644e%2Fen%2Fug.pdf&productCode=&highlightinfo=106955230,537,548

Dear
To scan a legal size, use ADF face upward also if you are using smar thru setting to legal size, then scan.
Thanks.

I have a Brother MFC-8840Dn (full legal-size flatbed as well as document feeder). I use Windows XP Pro and a USB connection. I'm not sure my fix works with other Brother models or different Windows operating systems, but it's a way to start troubleshooting at least.

I've always been able to print legal-sized pages from word processing or spreadsheet programs and receive legal-sized faxes. But I never could make legal-sized photocopies or PDF scans. The image was always cut off at Letter size. The fix is a Brother Utility set-up setting:

*** Go to Start / All Programs / Brother / MFL-Pro Suite MFC 8840Dn / Scanner Utility. (This link may also be called something like Scanner Settings.)
*** At the "Maximum Scanning Size", select "Legal" instead of "Letter/A4".
*** Click OK and restart your PC for the new setting to take effect.

It's like magic -- I can now scan a full legal-sized PDF, whether on the flatbed or through the ADF. I also tried a compressed TIFF scan and it came out fine.

**** Make sure your software and/or Brother Control Center scanning configurations are set to Legal if that's what you want for the particular image scan. The Brother Utility setting simply enables the option of choosing the larger legal-sized paper, which then allows the full legal-sized image scan, You always have to set your software to make that choice, just as you would to make and print smaller images on Letter, A4, envelopes, etc. (NOTE: I always start scans through software on my PC, like Adobe Acrobat or PaperPort, and the Utility Setting fix is working fine. However, I just tried pressing the SCAN button on the front panel of the MFC itself with a legal-sized document on the flatbed and through the document feeder. I still got Letter-sized image scans. So, there must be another setting somewhere for starting scans directly from the MFC that I'll locate later.)

I was about to buy a different brand of scanner because of the frustration with legal-sized images. It's like having a new scanner now :) I hope this Utility Setting info helps you troubleshoot your legal-sized image problem.

1. From the MP Navigator screen, select "Scan with Scanner Driver" in lower left corner.
2. When in ScanGear MP, select the Advanced Mode tab
3. At Input Settings, select Document (ADF) for document type
4. Set paper size in next window
5. Click Scan button at bottom

Using SCAN feature of Paper Port software - (after clicking SCAN button ... select Define under Options section of the window ... the option to select paper size then exists). With harmony.

Software issue for the printer. Uninstall driver and reinstall. If this does not solve it, it may be time for a new printer. Lexmark printers do not have a long service life.