My advice would be to use the smallest blade that gets the job done, not the largest blade that you think you can handle. Too many let their ego dictate paddle blade size (and they slow down dramatically toward the end of race).
Look for a good coach. Doing so can save years of trial and error. Otherwise video yourself often and compare to the sequences you see in a video (such as the Forward Stroke video by Barton/Chalupsky).
