Excellent advice here... just to add, I think you'd be fine on a 252 waist width or possibly a 250 depending on your stance as I've used a 252 width with size 11.5 boots and a 21" stance with 15, -6 angles with no problems. At your height you'd probably be using at least a 22" stance.
The difference between intermediate boards and beginner boards that would hinder your learning would be stiffness/flex (where the board be alot more sensitive to every input you give it) & bevel (a 90, or 0 bevel would also be more sensitive). At this point you don't need to worry about sidecut, shape (twin, directional), base.
In summary, if you're looking for a board RIGHT NOW I'd suggest:
- AT LEAST something 40% off, in the $150-$300 range
- 158-164 in length
- 252 waist or bigger
- mid-level flex (check the websites and compare boards), which usually means an all-mountain or park board
Specific suggestions off the top of my head are 159 Atomic Pivots and Axums on Ebay right now that are going for around $180 shipped.