Picking the right windsurf board size is tricky, especially if you're new to windsurfing.
Part of the reason it's tricky is because experienced windsurfers and the windsurfing media are a bad influence. We're always emphasizing progression to smaller boards, and dismissing big boards as just the "training wheels" of the sport. As a result, new windsurfers often end up with crazy misconceptions about what kind of gear to buy and what kind of path of progression to expect.
Here are some of the WORST pieces of advice that I typically hear:
Bad Advice #1- "I learned on XYZ terrible too-small board in the '80s, and now in PQR super-windy place where I live I never sail anything bigger than 85 liters, so if you're athletic and determined like me you should have no problem learning on this tiny shortboard that I want to sell you to use in your glassy calm 1-acre lake in Ohio."
Bad Advice #2- "Once you have a week or so of experience, you'll never again have use for a big longboard with a daggerboard, so you should just skip the longboard and buy this shortboard that doesn't work worth a crap unless the wind is blowing more than 15 knots."
Bad Advice #3- "If you want to learn to go fast and use the harness and footstraps and do jibes and waterstarts then you need to buy this tiny shortboard that will actually be impossible to sail unless you already know how to do all that stuff."
To counter some of that bad advice there is my Windsurfing Gear Calculator, which takes weight, wind strength, and skill level into account to figure out roughly what size and style of gear is appropriate. The calculator is a bit technical, though, so I've also tried to capture the essence of things in these two pictures:
This picture explains when you should use a big board, and when you should use a small board. Basically, small boards are only appropriate if you are a skilled windsurfer AND it's windy. When the wind is too light to plane, the best performing board will always be a big board with a daggerboard, and this is true regardless of the rider's skill.
This picture explains how board size is related to body size. What is a "big board" or a "small board" to you is relative to your body size, but the daggerboard also plays a role. I.e., a big person's small board would not be a good small person's big board, because it wouldn't have a daggerboard.