I have been using a small Bamboo on a 42" screen at a resolution of 1920x1080 for the past year as a substitute for practically all mouse input and I haven't had any problems with it. It may take slightly more pen control, and perhaps a better understanding of the importance of hot key zoom functionality, than mapping a similar resolution to a larger tablet, but it works incredibly well for everything I do, from Aperture to SketchUp to Photoshop and Illustrator.
Do I want a larger tablet? Sure, I'd love to have a new toy, especially if it were a larger Intuous or the new touch-capability Wacom. But do I need a larger tablet? Not at all; what I have now works perfectly for what I need.
I'd really suggest starting out with a small until you really understand whether or not you really need a larger size. There's a refurbished Bamboo Small on Amazon for $45...thats a helluva deal, and while it may not have the touch input you had originally mentioned, its a great way to dip your toe in without having a huge impact on your wallet.