IMO, there is no such thing as an average wind speed in St. Pete.
This place is all over the charts, even in one day... with today and yesterday and tomorrow being classic examples. Cold front days tend to do the same, going from 20 to 35 often all in the same day. Or starting NE real strong in the morning and tapering down to the low teens by 10am.

Yesterday was 12 to 15 in the morning, then 15 to 20 for a few hours and then 25 to 30 plus as the sun set. Many riders that arrived at EB early yesterday and stayed all day, like me, were all over the place with kite sizes due to the wind changing so much. Yesterday was typical St. Pete in my mind. Our conditions heavily affected my choice of kite brands this season because I just got sick of all the re-rigging I have done in the past.

St. Pete might have a statistical average over the past 5 years but nothing close to a true mode (most often occurring) wind speed.
You're most likely going to need a 16, 12, and 9 ultimately to cover all the bases.