x1000 on the demo suggestion! I never buy a board I haven't ridden. Now is actually a great time to buy older boards because the 2012 stuff just came out. No need to wait until summer. Size is actually fairly important...
-change direction quicker (this can be good for an advanced rider but scary for a beginner)
-are lighter (this is an advantage, but not very noticeable)
-land harder (this can put more strain on your knees)
-respond more sluggishly (this can be frustrating for an advanced rider but reassuring for a beginner)
-are heavier (again, you generally won't notice the difference while you're riding)
-land softer (spreading out the shock of a landing over a wider surface area transfers less force to your joints)
If you ride a board that isn't big enough to properly float your weight, the board will ride lower in the water than it's designed to. If you ride a board that's too big for your weight, it will tend to ride higher in the water than it's supposed to. Both of these situations are less than ideal because board companies design boards to ride at a specific depth range that makes their shapes most effective. Most boards come in a small size (usually 133-136), a medium size (137-140) and a big size (141+). A 135 cm board is the right size for someone who weighs 150 lbs. You could get away with riding the next size up, but I wouldn't if I were you.