Hi.

The best board in terms of dimensions (not thinking about brand at this stage) will depend on a few things:

1) Your weight

2) Your balance/board riding experience

3) What you intend to use the board for (flat water, open water, surfing etc), which you've answered.

Probably you'll best be suited by something about 10'6, give or take an inch, but the width will depend on the first two points. Generally 30 inches to 32 inches is about right.

All the major brands make a board or two this size, and they're all pretty good these days, so just depends what sort of deal you can get in terms of what brand to buy.