Any coldwater fish is messy, and bass are no exception to this rule. Both species can top out between 12-18 inches (maybe a little larger), with a largemouth getting a little heavier due to body shape.
125 gallons would be okay for either, I would think, as long as it was a single-fish tank (which it will be anyhow, since both species will eat whatever will fit into their rather ample mouths.).
These guys tend to hang out either singly or in small groups, so keeping one alone isn't a big deal. If you get them very young, you should be able to train them to pellets or frozen food, however, an older fish will be difficult to get away from live feeders.
They like very cool water, in the low-to-mid 60's, but they can adapt to low 70's with no real problems. Higher temps will really bother them, though, so a tank in a cool place, like your basement, would be the best situation if you don't want to invest in a chiller. The main problem with the warmer temps is the fact that they are messy, and are even messier if they are more active. You'll be plagued with ammonia issues if you keep these beauties in 75 degrees...
Bass are really a great fish, if you have the resources and time to deal with them. Just be sure you NEVER release the fish back into the wild if you decide they're not for you. My advice is do LOTS of research before you buy or catch one (always being sure you're not doing something unlawful), just to make sure the species is for you before you are faced with having to place it in another home, a neighbor's pond (with permission, of course), buying a bigger tank, or having to euthanize it....of course, bass are awfully nice on a dinnerplate (although I don't think I could eat a pet, personally...)
Good luck, and sorry for the long boring post...
Hope this helps.