I think the size preference for tablets depends on how the artist works. If you're used to drawing large and using your whole arm for expressive strokes, then obviously a larger tablet is going to be better for you. If you're used to drawing smaller and can get expression by just using your wrist, then you're better off with a smaller tablet (you're likely to not use all the space available, so why spend more?)

The other factors besides that really are cost (obviously the smaller ones are cheaper), and physical space available in your work area (a huge tablet might not work so great around a cluttered desk and the smaller sizes can even be held like a clip-board without getting unsteady).

I think I'd go up a size from the 4x5 (6x8?) and to an Intuos if I had the money for it. Just so I'd have some more stylus pressure range and a bit more room to work with. The 4x5 graphire is still pretty good for starters, and it will give you an idea whether you're even comfortable using a tablet without having to spend over $100.

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
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There might be something worth downloading.