A couple of comments.
First, I question putting ships like the Excelsior and the Constitution in the same category. The differances in size, capabilities and tech are more than a little major.
Second, the range of sizes / capabilities we see for Science Ships makes this category almost meaningless.
Thord, what about conversions? We saw the USS Brattain, a Miranda class, roughly equivalent to the Enterprise class, that had clearly been converrted to a pure science ship (with considerably reduced crew).
If you gotta do a classification system, I suspect it might have to be at least
two-tiered - one tier for hull size, then a variety of classifications and sub-classifications to cover the ship's general purpose and capabilities. KInd of like the ship classification system in 'High Guard' (the Navy book for the old 'Traveller' role-playing game).