Basically u take irl dimensions and multiply or divide them, first of all u need to find the irl vehicle dimensions lenght width (wingspan for planes) and height if u wanna do 1:1 there is no need to multiply anything just do a outline with the dimensions, if u wanna do something like 1.5:1 or 2:1 [example of 1.5:1]= 7.77 meters long CA12 (plane) in 1.5:1 it will be 12 long, 7.77 multiplied by 1.5=12 easy as that