I have an Intuos3 A5 Wide - Special Edition. (http://www.wacom-shop.net/cgi-bin/wacom ... Z&2D631WSE
Combine this with Corel Painter and you have a very powerful natural media simulation.
For retouching photographs though, a Bamboo will do very well (of course combined with Adobe Photoshop).
It takes some time to get used to it especially for drawing. It feels quite unnatural to draw or whatever and not look at the result appearing underneath your hand but on the screen. I use my tablet with a laptop, so I would often place the tablet on the keyboard and align and centre it to the screen. It is far more intuitive this way, but I cannot access my keyboard hot-keys
Now, when it comes to tablet size I find that even the smallest tablets available can be very very useful and even more convenient to use. I cannot tell much about photo retouching because I do not use my tablet for that but if I were to chose a new tablet I would grab a few sheets of paper of various sizes and I would simply scribble something, make a few of my typical strokes and see how comfortable I was, if space was insufficient or too much white space remained (tablet surface is expensive!).
Tablet surface gets mapped to your screen, so the closest in size and aspect ratio those are, the better.
If you are really passionate about retouching your photos, then the best solution in my view will be a big screen (say 20"-22") and an A3 or XL tablet (like Intous3 A3 or Intous4 XL - both will give you more than you need in still imaging; to the point that you'll be able to tweak every pixel without having to zoom in and out much).
Last but not least, the mouse is still a great pointing device.