In the model's folder there's a file named "model.siz" Open it with any text editing program. Inside you'll see entries that look like this->
ASSETS\MODELS\FCA\FCA.MOD 8.47
ASSETS\MODELS\FCA\FCA_BRK.MOD 8.44
Find a stock model (probably a base would be best ;)) and make the entry for the custom model the same size. The game will resize the model to the correct dimension. You can also play with this to rescale other things to your taste.
Well, if you have the directory correct, then I don't know. I've resized cruisers down to PF size. most of my model.siz file has been changed. The pirate models are too big in the stock game, IMO, just as an example.
For example, assets\models\custom base\custom base.mod 50.00
assets\models\custom base\custom base_brk.mod 50.00
Just relpace "custom base" with the name of the folder that the base model is actually in and use the correct name of the .mod file. I'm using "custom base" as an example. Make sure that the spacing is correct and that there are no typos. The numeric value for the size is in it's own column. IIRC, you just hit tab after typing in the directory then insert the size.