Jonathan Tagle says that when you can do EVERYTHING proficiently on the A-license card for canopy work, then bring it down a size (if you want).
Chances are you will be able to come down the first couple sizes fairly quickly, so if you purchase a 235 now, you might be regretting it in a few dozen jumps. It will probably end up saving you money to rent for a little while, see what you're comfortable with, then buy a reasonably sized canopy.