Which board size???

[jojoseez222]

Hey guys I'm new here and would appreciate any suggestions provided. I am new to the surfing thing, but have been out quite a few times since i moved to FL 4 months ago. I want to buy my own board now so that i don't have to rent them anymore and have found that i really enjoy it. I am a 6 ft 200lb guy all muscle (not bragging just saying cuz muscle weighs more than fat lol) and am pretty heavy. I rode a longboard 9' 2" the other day on some powerful 4ft waves at ponce inlet and wished i had something shorter because it was burying the nose unless i scooted way back. I have been using my friends 6' 10" (this design... http://www.tomneilsonshapes.com/surfboards/superfly.php) about 2 in thick sometimes but i feel as if i am sinking the board quite a bit and only the nose had about a foot out of the water. What size board should i get if i want to catch alot of waves and get good at turning etc. ? Also, should i be sinking the board much or at all? Thanks in advance!