How do I change the font size on a Dymo Letra Tag?

For the Dymo LetraTag Plus LT-100T, Personal Label Maker (SKU: 1733013)

Before trying to change the FONT you must make sure that the labelmaker is configured to print only 1 line of text. You cannot sellect to print larger fonts when label maker is configured for multi-lines. In order to make sure you are printing only 1 line do the following
1)While pressing down on the Shift key (may be show by an up arrow on some machines), press the Del/Clear key. This will clear all text and reset the label maker to print on 1 line.
You must also make sure that there are no other print options enabled, such as: italic, bold, vertical, underline, mirrored-print, outline, etc; these will all prevent you from changing the font. Also ensure that the take will accomodate the font.

My response will depond on the exact model you have...
To set the font size
1) Press the FORMAT key.
2) Use the up or down arrows keys to select the Size and press OK
3) Use the up or down arrow keys to selected the desired font size then press OK

To set the font
1)Press FORMAT
2)Use the up or down arrow key to select the font style and press OK.
If this does not work, please include the exact model/name.