I’d go with size, or more specifically mass.
I guess my problem with that is that we're pretty explicitly talking about a class of warp drive / stardrive, not class of ship.
If Class 6 meant engines capable of moving ships in the size range 6, why not speak of the Class 6 of ship size instead? Apparently, a mere knowledge of the ship herself is not yet sufficient for establishing the Class of her warp drive, there being this additional need to say "Ours has Class 9" as if there were multiple alternatives.
Also, when the "Class 7" issue arises, in "Valiant", this appears to be a specific design or a design family of engines on which an engineer may or may not have competency, rather than a matter of output range or other parameters. The narrower the definition, the more reasonable the dialogue over whether Nog knows how to run one or not. This might support the interpretation that higher Class is a newer design, even though the shuttles do it better (in a fashion more naturally fitting the English language) by using Type in that sense. Yet just as with shuttles, one would run into the issue of there being ridiculously few Classes out there: surely Starfleet, enamored with countless parallel ship or shuttle designs per era, would already be up to Class 47 at the very least. Or, alternately, a given Class would cover dozens of dissimilar designs and be meaningless.
Again going back to "Valiant", Nog says that the
Defiant, too, has a Class 7 (additionally specified as "nearly identical" to the
Valiant one), as if this were news. If two externally identical ships aren't expected to have the same Class of warp drive by default, we need all the more specificity to the term. Should we think that the surprise factor here is that both the ships have been overengined above their blatantly apparent size range, a rare thing both sets of engineers take pride on, and the secrecy surrounding the
Defiant class confuses both as to how rare this in fact is? Or that both for their part realize it's unusual to have a Class as new as 7 on a hull that really is a 6 by age (both ships being long-mothballed failed prototypes)? The latter would nail down Kirk's ship as a Class 5, so that she can still gloat over a Class 4 in "Bread and Circuses" - but with zero wiggle room toward either the obsolete or the futuristic, which isn't all that comfortable.
As a pure apropos, the late VOY tendency to generically refer to shuttles of many distinct Types as "Class 2" could be thrown in: the ascending order of performance would be fine for explaining this Class 2 as a warp engine "size", while OTOH completely incompatible with the idea that Class would denote age of the design. Kirk's ship could be humiliatingly higher up than Class 4, and the
Beagle in turn humiliatingly close to a mere shuttle...
Timo Saloniemi