Yeah, but this is ineffective for several reasons:
a) SA cars are to big.
b) SA cars have different wheel sizes
c) the new modcar may have a completely different wheel size compared to a sa car (you can't compare a 17" wheel luxury car with a 13" mini car like VW Golf)
Get it ingame, shoot screenshots from a 90° angle of CJ next to the car from different views and get CJ in mind on about 1,8 meters (compare to doors heigths, average ped height, etc). Shoot from behind, side, front .. and google for real life pictures with people of about 1,8 next to a similar car. If you find only some with strange angles, replicate these angles ingame with CJ and your car.
Once you have enough to get a good comparison, finetune the scale of your car TO THOSE SCREENSHOTS and PHOTOS.
For example: a Maybach would be higher than CJs nipples, a ferrari or lambo would be around his belly button, while a BMW M3 could be just under his breast. And don't be afraid it could be too small: check it again with different real life pictures, you might be surprised.