A ship of the line was one powerful enough to take a place in the line of battle. (The term "battleship" has a similar etymology.) Generally, this meant a 3rd Rate or larger ship, with guns on two or more decks. Rated ships smaller than this were frigates, and carried all their guns on a single upper deck.

Rate

Guns

Men

1st Rate

100 or more

875 to 850

2nd Rate

90 to 98

700 to 750

3rd Rate

64 to 80

500 to 650

4th Rate

50 to 60

320 to 420

5th Rate

32 to 40

200 to 300

6th Rate

20 to 28

140 to 200

Sloops

16 to 18

90 to 125

Gun-Brigs
& Cutters

6 to 14

5 to 25

The number of crew on a 1st Rate was increased by 25 when used as an Admiral's flagship, by 20 with a Vice Admiral and 15 with a Rear Admiral.

Originally from http://www.cronab.demon.co.uk/gen1.htm, with the author's permission.