TeamVMG is quite right. The mast on a 16 is very strong but ALWAYS keep plenty of mainsheet tension to act as a rear stay. Do you have any spinnakers available? I would just use whatever you can get your hands on. I found the 21mt kite as easy as any to use. It seemed to have the power to keep the boat steady whereas the smaller ones let the boat stop and start in the waves. Dont worry too much about it. Just get whatever is available and start having fun. Then decide later which one might suit best.