How to choose tablet size?

I mainly do webdesign, but as I’m starting to move more towards custom graphics and illustrations, I’m thinking of buying a tablet.

I’ve nailed it down to Wacom Intuos4, since it’s possible to use both left and right handed, but I’m not sure which size to choose.

The two I can’t decide between is Large (A4) and Extra Large (A3). I think that having the extra space of A3 would be useful, but it also costs almost twice as much as the A4 version.

What I would like to ask is, do I need that extra space that A3 provides? Is it such a big deal to pay twice as much, or should I start with the “smaller” one and then eventually upgrade when I outgrow it?