Here is a good read... and then some info I pulled from the "Deviant Art" forum.
http://mousevomit.com/tabletfaq.htmlWhat size?
Tablet size often correlates to your screen size. A small tablet on a huge monitor is going to be no good, as your hand movements will be too large for accurate drawing. A huge tablet on a small monitor is almost as pointless, although good for detail work, and very expensive.
Also think if you want it to be portable or not - an A4 (9x12") tablet isn't going anywhere but a desk!
Size also correlates to your hand gestures. Do you draw small and detailed? If so, get a smaller tablet. Do you huge large, sweeping movements? A bigger tablet is what you want!
Finally, aspect ratios. If you have a widescreen monitor, seriously consider Wacom's new range of Intuos3 Wide tablets, which are mapped for the 16:9 ratio of widescreen displays.
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"I couldn't find the sports car of my dreams, so I built it myself." - Dr. Ferry Porsche