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The number of teeth is for choose the sprockets...an it vary according to the primitive diameter of the sprockets!
The belts that you need are all GT2 wide 6mm, you can buy some meters for few bucks!

Coming back to the lenght problem, the body can be shortened few mm (I can suppose around 4 mm, that means you will save around 8mm of belt in total). Anyway, you can find some remixes a little shorter than my model.