well,people say wacom's are the best. I personally have tried a genius (cheaper brand) and now i have a wacom (worked hard the summer to buy it).
Dont look to buy bigger.Size doesnt matter.Big tablets are uncomfortable to use.Genius is not a bad brand,but it has its cons:
*the pen works on normal batteries,which you have to change, while wacom's pen's batteries are recharged by the surface as you draw (no need to change...ever)
*genius might have more pressure sensitivity,but it actually wears you off,as you have to press the pen harder...this wears off the wrist quickly and makes it uncomfortable to use...when as with wacom you dont have to press it.It runs smoothly..
I have a wacom graphire 4 classic XL (6x8,a5)http://www.computerarts.co.uk/__data/assets/image/394082/art117revwacom450.jpg
I think it was well worth it
If you want to buy something cheaper,and if it is going to be for photo manipulation (not actual drawing), I'd suggest you try the volito series.I hear good things about them.