There was a recent talk in the GCTalk forum about this. The consensus was that the 6x8 was the best, unless you plan to remap some area of the palette. None of the users who had a 8x12 said that they used fully, they all remaped the excess area.
If you can afford it, go for the intuos, I have tried teh Graphire and I prefered the Intuos. It gave my sketch a more 'pencil' look, but the difference is very small so dont feel bad to take a Graphire!
BTW, it take a while to get used to the tablet so try using it as much as you can, you will improve very quickly.