Matt,
I've found myself in a similiar situation.
I weigh around 100kgs and have only just started wind surfing a few months ago, After using the initial 200+ litre hire boards I moved to a JP excite ride 150L FWS (bought second hand at a bargain) which volume wise I found great. But I've since dinged and damaged it through the learning process. I intially thought it would be too big but it turned out to be a great board.
In my limited experience I'd
not only be looking for the right volume but more importantly a tough and durable board, because as a beginner you'll bang it up. (repairs can be costly

) As a beginner I've wanted to progress as fast as possible, it hasn't helped being in the repair shop for a week at a time. I've since traded the X-cite ride and gone for a Starboard Go 144. (bought yesterday...will hit bay this wk end...hardest part over...girlfriend's been won over by the colour...


)
Below is what I put my issues down to:
Issues: 125-130L can still uphaul but could feel unstable under your feet and may sink a little below water level. But once yr a confident sailor will probably not notice it (may take time

). This is dependant on yr skill level and this will intially depend on how consistently & often you can hit the water.
140-150L easy to uphaul extremely stable and can handle larger sail.
The important thing is you want to consistently enjoy yr sailing. My experience has been that light winds are fine but once the wind picks up technique becomes critical, a STRONG & DURABLE board will allow you to progress quicker and be more confident in trying stuff. There's nothing worse than waiting for the right conditions to come along, then hitting the water and smashing your board up and having to sit on the shore watching everyone else blasting, if this happens to many times, windsurfing becomes wind suffering and it can become all to difficult and you just give up and take up an easier sport.
Solution:Whatever volume works out right for you, bye a strong, durable board. Its about havin fun and staying on the water.