Do you find that you have more control on the wave with a board that size or one that is smaller? I have been SUPing for a while and want to start taking on the waves.
If I understand your question, you want to know what size of a board may work for you best in the surf - provide stability, maneuverability etc.....
If so, I would say that the best thing you could do to select the right board for the surf is to have an experienced rider give you some advice based on your weight, fitness, sup experience, surfing ability and the type of break you would be mainly surfing (beach break, reef, point etc.).
The answers to those questions as well as how you would like to progress would provide the framework to give you some options as to what equipment you should consider (template, dimensions etc.).
As for myself, this shape is pretty agressive, low volume (about 105 liters) which is not a lot considering I am 215 lbs.
This shape is very loose and can be modified for a variety of surf when modifying the fin set up. I surf this board mainly as a tri-fin (with the trailing fin being smaller than the front two). I cannot see myself going much smaller at my weight as this board is already under water when stationary. Having said that, I can control this board very well and pretty much make it do whatever I want it to (within the conditions it was designed for).
I am sure your adventures in SUP surfing wil be rewarding. If you are an experienced surfer then you already know how to handle yourself with others in the line up. If you are new to surfing I would suggest learning important safety issues regarding the equipment, water conditions, and the safety of others. Additionally , beefing up on surf etiquette is a must.
Hopefully this info. was useful and feel free to contact us if we can be of more assistance.