It is not quite clear to me from your question whether you expect / hope to full time in the trailer or just some-time in it. Makes a big difference. And there's a LOT more to the equation than just length of the coach.
As someone noted, the absoulte best thing you can do is to go to a nearby rally and take as many tours of different coaches with different configurations as you can (nearly everyone will offer you a tour - really!). Then you can see, touch, experience what the interior differences are. This will help you decide what's important for YOU!
I usually travel solo, so my 23 footer is - to me - ideal. When I get to / have to share the double bed, it gets real cozy real fast. Just back from a six week trip with my brother, and he slept on the front gaucho. That works fine for a guest, but a few weeks of making up the bed every night gets old pretty quickly also. So a lot depends upon how much space you take up, how often you expect to have guests, and how "friendly" they will be.
In general, if you are a.) gonna' be in one place for long periods of time, or b.) encounter lots of rainy / poor weather, or c.) parking space size is not an issue, then bigger is better, but often requires a stouter, more powerful tow vehicle and the larger coaches also won't fit into quite a few sites in older campgrounds. On the other hand, if you a.) plan to be in State / Federal parks a lot and/or b.) expect to be moving around quite a bit, and c.) seldom have overnight guests, then smaller is better and can be towed with a wider variety of tow machines. When you look, pay close attention to storage space, counter area (makes a BIG differene for cooking), and size and location of refrigerator and bed.
There are several other dichotomies. One is single axle vs. multiple. Several of those posting earlier have noted the plus and minus sides here. To me, multiple axles are a significant safety factor. But that also makes for more expense at tire and axle replacement time and more maintenance for bearings and brakes. Another fork in the road is newer vs. older. In general, the older rigs are lighter in weight and have somewhat simpler systems. If they've been well taken care of they can be a real bargain. If not, they can be endless repair and replace work. On the other hand, newer may well mean appliances, axles, etc. that you can use longer before replacing them and possibly floors in better shape - with less to no damage from water leaking inside.
A final note: there aren't too many real "bargains" out there. Assuming that there isn't some hidden damage (do get a good inspection!), a coach in good repair will depreciate fairly quickly from the day it's taken from the dealer's lot and then will level off and hold its value pretty well over a long period of time. So if you come across a rig where the price seems "too good to be true" - it could well be a scam. Keep you eye on the classifieds and watch your local sales ads and you'll quickly get a pretty good idea of what to expect.
Good luck to you!