Might be worth trying it with 15' at first to see how it performs. Depending on the adjacent 2nd story and wind orientation it might be a problem or maybe not. This could also be affected by terrain. Highbeam has his Princess on 12' and it works ok, so maybe that is another option?
and get a nice flashlight and look inside and do a visual inspection. If your roof can be walked on and you feel comfortable doing it also. Also if your just buying the house it have your inspector get up top and tell you the condition of it.