The absolute maximum Teeth I would suggest is for 84T for 48P Gears. This is almost flush to the bottom of the chassis with the "2.5mm chassis"
Outside Diameter = ( Number of Teeth +2 ) / Diametral Pitch
So... with the maximum gear we know which is 48P 84T
Outside Diameter = ( 84 + 2 ) / 48 = 1.791in (This is your limit...)
This is the theoretical value. so check the diameter of a brand new gear you have and calculate. And always keep in mind the difference as that will be your error %.
also when you do not know the maximum teeth that can be used, measure the distance form the center of the drive shaft to the bottom of the chassis which is the radius of your gear then you can calculate...