If you want a nice balance between size, quality and cost, may I suggest the Wacom Bamboo Fun Pen and Touch in A5. You'll most likely find that unless you're looking to do really large work that an A5 will do most jobs very well indeed. A6 is nice and cheap, but you'll most likely find that the drawing area just isn't sufficient, and most A4 will either set you back financially a very long way or be of poor quality.
I've got the A5 Pen and Touch and it is very good, the pen is passive and as such requires no batteries as it is the board that does the sensory work, so you won't have to keep replacing things or worry about losing power mid drawing. The sensitivity is fantastic and unless you're setting out to do professional work (and since you're asking for opinions on what to get I am assuming you aren't, at least not yet) then it will be a great buy. A5 is, for me, the best compromise for size unless you had about £300-400 spare for an A4 of good quality.
You also get lots of free software with it, including Photoshop Elements 7 and a couple of other programs, so you pay about £170 for a great quality tablet and maybe £100 worth of software to boot.
Happily recommend it.