Would there be different crew member sizes? Number of broadside cannons, mortars, bow cannons, etc? Different ranks in crew members (such as different ones on one rate, but not on another rate)?
In the British navy of the 18th and 19th centuries, ships were rated by the number of carriage mounted guns they carried. Pole mounted carronades, swivel guns or mortars were not counted. The more guns, the lower numbered rating it had, with first rate being the largest (this is also where the English phrases first rate and second rate come from when describing quality of an item). Of course, the larger the ship the larger crew it would accommodate.
A ship of the line specifically referred to the first through fourth rates, while fifth and sixth rates were classified as frigates. Ships with too few guns were unrated, and classified either as sloops or brigs.
Here is a list of ratings and their associated number of guns.
Sloops and Brigs:
Unrated: less than 20 guns
Frigates:
6th rate: 28 guns or less
5th rate: 44 guns or less
Ships of the Line:
4th rate: 60 guns or less
3rd rate: 80 guns or less
2nd rate: 98 guns or less
1st rate: 100 guns or more.
As for your question about rank, an unrated ship may be commanded by a commander, while a post-captain would be required for the command of a rated ship. Traditionally each rate would also have an associated number of lieutenants posted on the ship, with sixth rates having only two and first rates having six.
Hey jimmy_costigan and jschooltiger, would you like to see an earlier question of mine I posted? Some of it does revolve around naval history, maybe you two would like to check it out?