How do you know what size anchor a boat needs?

You can get a Ground Tackle Force Load sheet from here for free and it should help you figure out the size you will need.

What anchor does your boat need?

Based on your boat's unique characteristics, we can determine the true Ground Tackle Force Loads (GTFL), the nature forces including surge-forces, that your boat and mooring gears, will be exposed to, in different wind speeds (15, 30, 42, 60, 120 knots).

The exact GTFL is very valuable for any boat owner to know to understand your boat's unique characteristics, how it reacts to different wind speeds!

http://www.marine-boat-anchor.com/-10%20gtfl.htm

Hope This Helps.