dont be so concerned about loosing efficiency using a 21 inch. no offense but with as few yards you have, and with you only being 13 you should have plenty of time to get your yards done. im 15 and play football and i have practice everyday and sometimes on saturdays. The weekends is when i do most of my mowing, ive got about 10 yards all on the big side mostly. if i were you i would keep using the mower you have now and get some nice $$ in the bank so when you get more yards and you figure out what kind of mower you really need you can get yourself a nice piece of equipment.