The Galaxy class could be a fairly decent carrier. It has several shuttlebays and its main bay is fairly large, you could fit probably a fairly reasonable number of fighters in.
I believe you're right. The Enterprise-D brought DS9 their original runabouts. Now, I dunno if they were all stored in her main shuttlebay or had one in each bay (the math, I cannae do!), but a runabout is roughly twice the mass of a fighter... I would imagine that would mean a Galaxy class could carry at least six fighters, probably more if the other shuttlebays are used.
Ya, but the federation really needs to have a pure carrier to really be a carrier. It would be interesting if they build a ship like that and I hope if they do it does not look like the BSG ship.
If they did, it should be hmmm five times longer then the USS Enterprise E
I agree with Robot Elf, it doesn't have to be longer, as the Galaxy and the Sovereign already have that going for them. Rather, I imagine that all the Galaxy would need is some modification -- it's already got the internal space. If a Galaxy is fitted for wartime, it doesn't need so many observatories and science labs. It's been said that the Galaxy has modules that can be exchanged depending on the need of the its main mission.
All that space can go to building additional hangars and shuttlebays, I would imagine.
Same deal with the Nebula, whose top module comes in a variety of shapes, sizes, and purposes. I don't think it would be too hard to design a fighter-carrier add-on for the ship. Also, if the original plans for the Akira come through, the top rollbar itself serves as a pure shuttlebay like a carrier, with the rear of the rollbar as the Entrance and the front as the Exit.