hi, it really depends on how much experience you have, where you want to sail, and what type of windsurfing you want to get into.

for example, i've been windsurfing since May 09, and can quick tack, carve gybe and waterstart. I'm using a 105l board. It suits me well as I can get out in most conditions on it.
Basically the more bouyancy you have (in litres) the easier it is to windsurf on. (big generalisation)
You can generally uphaul on 100 litre+ boards in flat water tho it is not easy till 120+
In waves it is a lot harder to uphaul so on a 100l board you need to be waterstarting.

Basically what i'm trying to say is if you can give us some info on your experience we'd be able to help you more.