modding question-vehicle sizes

[Imperial]

Hi
Is it possible to mod a race in the following manner?
1) allow it to have less than the regular number of ship sizes--for example- If i wanted a race to have cruisers, battlecruisers, battleships, dreadnoughts and baseships only (and just these sizes) can this be done for just one race? (trying to see if I can have one race that does this and another that has the standard amount.

Thanks for any help--ideas--answers