Hi
I use an A4 Wacom - previously had A5 and have used A3 at work.
Firstly - graphics tablets are the way to go for any serious editing, and they are great fun to use to. (Don't let the kids near them as you won't get it back!)
Size wise - A3 is way too big unless you're going to be tracing drawings - it just takes up far too much desk space. A4 (mine) Works well for me. A5 was really good to. Going smaller, I think it depends how you are going to work. Obviously the larger the area, the more free you can be with your brush strokes.
But there is always a trade: the larger tablets take up more room - and for a messy oik like me that's always a problem!!
Go and try a couple - do you have a mac store near you?
Pete