Floyd is correct, but in very simple terms, fin size is determined by many, many variables.
Water state (chop)
Function (free sailing, waves, slalom, formula, freestyle, speed, etc.)
Best to talk to experienced windsurfers in your area or a shop. You can probably find used fins pretty cheap if you look around. For beginners, fin quality (expense) isn't a big deal. Getting the right size and design is important, but spending a lot of money is not needed at this time.
As your skills and experience improve, upgrading your fins will become more important.