To what tolerance are SIR's and TIR's measured? and with what tool?

The second tier of my inquiry is less IT related, but I know we have some good rules guys here including a couple ST guys...

And how is the size of the SIR/TIR and weight of the vehicle decided? Especially for something like all the GT2 vehicles like GT3 Cup cars, Ferrari Challenge, Audi R8 and many of the STO implants where the prep level of the engine is limited compared to the rest of the class.

TIA!