Hi there, welcome to the forums!
I use an Intuos 3 6x8 at home and personally find it a pretty good size for me, but at the end of the day I do believe this one comes down to personal preference. I used to have a 12 x 12 tablet which was like a big unweildy brick in my opinion but I believe a few users do prefer the large sizes. 6 x 8 does seem to be a pretty standard sort of size though in my experience.
With red eye reduction, photo editing and that sort of thing, part of it is going to come down to the features of the software you are using. having a pressure-sensitive pen-controlled cursor can be really handy for thinbgs like touch up and precision work, that's true. I find it gives me a lot more control than with a mouse.
I'm not an expert in tablet hardware, but I believe that the tablet itself does a lot of the work, a pen interacting with transducers in the tablet which measure pressure and if applicable, tilt, as well as the position, that sort of thing.