Frigate has always been more of a role than a size cateory. Traditionally, a frigate is the smallest vessel capable of operating autonomously and not as part of a fleet. This doesn't count point-A-to-point-B vessels like corvettes or shuttles, that can operate over long distances. I mean go out, and stay out doing things. They tend to be more single-purpose than a larger cruiser or ship-of-the-line. Destroyers tend to be gun platforms to augment fleet operations, and little else.
That last bit was a segue, by the way, to address the term "Star Destroyer". Some over the years have interpreted it as syonymous with "space destroyer", as in a destroyer-type ship in a space navy. These are also the folks who coined the term "star dreadnought" to refer to the Executor-class command ships. As far back as I can remember, though, I heard it as "destroyer of stars" -- the same sort of hyperbolic Imperial nomenclature that gave us the Death Star. Han specifically referred to them as "Imperial cruisers", and that's good enough for me. Cruisers are good multirole, independently-operating vessels, generally the next size tier up from frigates. So that gives us a good bookend the other way.
What is a frigate in Star Wars? Bigger than a corvette, smaller than a Star Destroyer.
--Jonah
