Had a mechanic fix my transmission on my 96 f15 xl and he over tighten the bolts now they just spin and won't come out so I was thinking about cutting off the heads of the bolts to pull out and replace them but will I have to go up one size bigger afterwards and if so what is the size because I'm trying to do it myself from now on when I can...

 

We need a little more information
What engine
What transmission (manual or automatic) and model if you know it
Which bolts were over tightened? Are these metric or SAE and what size.
My guess is that you are talking about pan bolts on an automatic.
BTW you can generally get striped bolts out by prying under the bolt head while turning the bolt counterclockwise

 

The bolt threads likely got wiped out when your mechanic was trying to remove bolts which had seized. You likely will have to re-tap new case threads.