How to add car models

Dear modders,

i would like to add car models but i don't find any information on how to do it.

I think it is like adding building models but i don't know how size works. what is the maximum dimension ? How can i specifiy the size of the car ?

If someone already did one (i don't find any for the moment) of know how to do it, i will appreciate

Regards,
Nico.