First you will need to check with your local building department as to your zoning for size, height and setback (distance from property lines) restrictions and if planning to have a bathroom and/or kitchen what septic requirements you will have. That aside I would then look at how I will be using the space? Do you want enough space to have an eating area or maybe couches and television? I usually like to make a space like this “convertible”, so the shape and size are useful for different uses so it can be changed as life changes. I also like to have a large openings leading out to the pool so when left open the space feels like and open roof like resorts have. And finally, I would always recommend a little larger than your original thoughts because the most common regret is, “the space could have been bigger.” I know I didn’t give you an actual size but I hope these ideas help.