WHAT SIZE BOlTS ARE REQUIRED FOR THE tHROUGH TOP CABINET CONNECTION ?

Whatever size matches threads in microwave.

1/4
1/4 x 28

They are 1/4' x 28 x 3 1/4" and this link will take you to GE and a set of pdf instructions if they are needed.
http://products.geappliances.com/MarketingObjectRetrieval/Dispatcher?RequestType=PDF&Name=49-40409-1.PDF

mcdevito75 here, Best Bet bring the mounting bracket and bolts recieved with the unit to your local hardware store. they will give you the correct bolts need to mount undera cabinet.

There should be two or three bolts inside of the cabinet above the microwave. Remove all bolts and then pull top edge of microwave forward (away from wall). It pivots on a mounting plate which is secured on the wall. Pivot forward and lift microwave up a little bit off of the ledge of the rear mounting plate.

Make sure to feed the electric wire (plug) through the hole in the cabinet as you pivot and remove the microwave. Sometimes cooking grease will make the microwave stick to the cabinet. So after removing the bolts use gentle force when pulling on the microwave.
I hope this helps and good Luck!

Jim here. In cabinet above microwave, locate 2 mounting bolts.
Place a box on the stove top to support the microwave once the mounting bolts are removed (you may wish to cover the box with a thick towel or small throw rug).
Procedure:
1. Unplug microwave.
2. Remove both mounting bolts from top of microwave (they secure it to the overhead cabinet).
3. With microwave front angled down onto box, pull some of power cord out of hole (to provide some moving space).
4. With microwave hand supported, remove box from stove top (a second set of hands would be helpful here).
5. HARD PART- detach top of microwave back from wall mounting plate by placing a hand under the bottom rear of the microwave and pushing up. (Wall mounting plate is screwed into the wall behind the microwave and provides both top and bottom support to the back of the microwave.)
6. Lower front of microwave until you can see about a 1-inch clearance between the top-rear of the microwave and the cabinet above it.
7. Firmly lift the microwave straight up and then bring forward. It should be free of the wall mounting plate.

i had the same trouble, yes i used a solid bolt to hold the microwave bracket in the stud then used the biggest toggle bolts that will fit, also the cabinet will not have to be changed. it works fine, if you had a vent a hood dont forget to hook up the exhaust vent. i figure your old exhaust vent is in the cabinet like mine was

Her is a link that I hope with work that shows the bracket and how it was installed.

PDF]  JVM1850BH/CH/SH/WH – GE Spacemaker Over-the-Range Microwave Oven File Format: PDF/Adobe Acrobat - View as HTML

it depends on your microwave and the kind of cabinet you have planned to install this let me know .. so i can help you further

I think there are only three very long bolts that can be used to attach this microwave to the cabinet floor or shelf. Grease, paint, varnish all can make the case stick to the cabinet. You might try running a putty knife or scraper around all the exposed edges keeping the top bolts in place so it will not fall down.