Here is a version at the matched saucer widths. I think you're right for purposes of scaling, it's close enough.
From a designers intent standpoint, an excerpt from Memory Alpha which has an excerpt from The Star Trek Magazine:
Regarding "outgunning and outrunning":
In TOS there is the Orion small scout ship in "Journey To Babel" that was configured on a suicide mission to destroy the Enterprise. The ship's power output was so high that her phaser strikes were more powerful than the Enterprise's and still be able to outrun the Enterprise. So we know from precedent that power output, not size determines firepower.
But, we do not know in TWOK whether the Reliant or the Enterprise is more powerful in an undamaged state. We do know that in "The Search for Spock" that the Enterprise held speed records that Stiles wanted to break, so odds are that the Enterprise (when not crippled) is faster than Reliant (when not crippled).

From a designers intent standpoint, an excerpt from Memory Alpha which has an excerpt from The Star Trek Magazine:
..the USS Reliant, was designed at Paramount by Joe Jennings, Mike Minor and Lee Cole. On the design Jennings noted,"It was the first time a new spaceship had been designed since the Klingon battle cruiser. That was a lot of fun. She was supposed to be a coastal and geodetic survey ship, like a buoy tender. She would be armed perhaps, but only lightly; she wasn't a lion ship like the Enterprise. Also, remember, the Enterprise was supposed to be an exploratory vessel, where the armament was secondary. That was even more true for the Reliant; she was supposed to just stick around in the known universe and take care of things that everybody already knew about.(...)It was fun to try to make it look identifiable different; we had long postulated that the circular saucer said "This is Starfleet Navy," and it uses engines that looked pretty much like those on the Enterprise."(Star Trek: The Magazine Volume 3, Issue 5, page 69-70)
And about ship sizes from alt.tv.star-trek.ds9:
Chris wrote:
> Well, I am looking for some ship size info.. I know that the Enterprise is
> about (or exactly) 641m long, and the Borg ship is about 3 times that at
> 2000m (all dimentions).. I'd really like to know how USS Voyager, USS
> Defiant and the two known Jem Ha'dar ship classes (the fighter and the
> battleship) compare to those sizes. I would imagine that all of the latter
> are smaller then the Enterprise however the Jem Hadar battleship could come
> close? Any info on that at all?
> Trypt
Akira = 860'
Sabre = 625'
Steamrunner = 800'
Vor'Cha = 1500'
Neg'Var = 2250'
Jem Hadar Battle Cruiser = 2500'
Jem Hadar "Bug" Fighter = 500'
Cardassian "Hediki" Fighter = 500'
Voyager = 1200' to 1250'
Defiant = 560'
Small Bord of Prey = 360'
Martok's Bird of Prey = 450'
Runabouts = 65'
DS-9 Station = 5280'
Reliant (Merada Class) = 500' to 560'
Some fans may argue these sizes with me...but these ARE what we use on
the Trek shows. You may get some variation in an episode in order to
satisfy a particular story point.
David Stipes, Visual Effects Supervisor, DS-9
EAS lists the Reliant as 243m. Memory-Alpha lists it at 233-234m. My model comes in at 242.7m with a matched beam and 227.6m with a matched bridge dome.> Well, I am looking for some ship size info.. I know that the Enterprise is
> about (or exactly) 641m long, and the Borg ship is about 3 times that at
> 2000m (all dimentions).. I'd really like to know how USS Voyager, USS
> Defiant and the two known Jem Ha'dar ship classes (the fighter and the
> battleship) compare to those sizes. I would imagine that all of the latter
> are smaller then the Enterprise however the Jem Hadar battleship could come
> close? Any info on that at all?
> Trypt
Akira = 860'
Sabre = 625'
Steamrunner = 800'
Vor'Cha = 1500'
Neg'Var = 2250'
Jem Hadar Battle Cruiser = 2500'
Jem Hadar "Bug" Fighter = 500'
Cardassian "Hediki" Fighter = 500'
Voyager = 1200' to 1250'
Defiant = 560'
Small Bord of Prey = 360'
Martok's Bird of Prey = 450'
Runabouts = 65'
DS-9 Station = 5280'
Reliant (Merada Class) = 500' to 560'
Some fans may argue these sizes with me...but these ARE what we use on
the Trek shows. You may get some variation in an episode in order to
satisfy a particular story point.
David Stipes, Visual Effects Supervisor, DS-9
Regarding "outgunning and outrunning":
In TOS there is the Orion small scout ship in "Journey To Babel" that was configured on a suicide mission to destroy the Enterprise. The ship's power output was so high that her phaser strikes were more powerful than the Enterprise's and still be able to outrun the Enterprise. So we know from precedent that power output, not size determines firepower.
But, we do not know in TWOK whether the Reliant or the Enterprise is more powerful in an undamaged state. We do know that in "The Search for Spock" that the Enterprise held speed records that Stiles wanted to break, so odds are that the Enterprise (when not crippled) is faster than Reliant (when not crippled).