Today I surfed small, slow waves and I often had to take a few steps forward on the board and by doing so I think the pressur on the fins reduced and the board sometimes wanted to go sideways down the wave.

I have a quad fin setup, but can convert to 2+1. I'm thinking of getting a larger middle fin and see if that solves the problem.

What is a normal size for the middle fin on a SUP? Can I use a wave fin for windsurfing? what's the difference between a windsurfing fin and a SUP fin?

A 7"-8" center fin is what you need.

A windsurfer fin (wave board) will be fine and there is no real diference between a windsurfer fin and a SUP board fin..IMO.

DJ