We know the Galaxy
is faster, from "Tin Man". Obviously, Romulans wouldn't volunteer the specs of the design, but by the time of the episode, Starfleet must have accommodated enough intel to justify this assessment, and indeed the episode supports it in the sense that a D'deridex
is badly damaged by forcing a speed higher than that of the E-D, and another warbird that doesn't want to be damaged is forced to travel slower than the E-D.
We know the D'deridex
was intended to be larger, and was occasionally photographed as being larger, but this isn't all that clear-cut. We have seen ship vs. ship photography in conflict with size estimates obtained from other and more objective visual comparisons (doorway sizes or whatnot). Usually, such conflict is associated with comparisons of Starfleet ships vs. cloaking-capable enemy ships; perhaps cloaks have a tendency to alter the apparent size of a ship even when idled?
We know little about firepower, as any situation where a single warbird actually destroys a Federation target or is briefly feared of being capable of this is a "pants-down" one, and any situation where Picard objectively believes he absolutely cannot win the fight even with a fully prepared Enterprise
involves more than one warbird...
So canonically it's sort of an even score, with one round undecided.