If the Romulan task force seen in "The Enterprise Incident" had been comprised of TOS-era Romulan warbirds instead of Klingon cruisers, I would definitely make sense for the warbirds to be the longer length. I suppose it might work, but the Romulan interiors in "Incident" were a bit spacious; this would be a little jarring after the submarine-style Romulan cockpit we saw in "Balance of Terror". Speaking of "Balance", I got the impression that the attacking Romulan vessel was that era's Romulan analogue for Sisko's Defiant: a compact, dedicated warship; so dedicated that it didn't make a decent starship.
Another handy semantic point should be remembered: the characters aboard the Enterprise did not exactly say the Romulan ship was operating under "impulse power". Instead, the word "impulse" has to be viewed in context:
SPOCK: Obviously, their weaponry is superior to ours, and they have a practical invisibility screen.
MCCOY: You're discussing tactics. Do you realise what this really comes down to? Millions and millions of lives hanging on what this vessel does.
SPOCK: Or on what this vessel fails to do, Doctor.
KIRK: Yes, well gentlemen, the question still remains. Can we engage them with a reasonable possibility of victory?
SCOTT: No question. Their power is simple impulse.
KIRK: Meaning we can outrun them?
STILES: To be used in chasing them or retreating, sir?
Note that Scott did not say "impulse power", but described the Romulan power as "simple impulse". Since the Enterprise's pursuit of the attacking ship obviously covered interstellar distances requiring (limited) warp speed, in was equally obvious that the Romulans were FTL, just not as advanced as the Enterprise.