I would disagree with 628. While I haven't sailed a Future 133, several years ago I raced a bic techno 283 which wasn't as wide as the futura. It came with a 48 fin, but I added a 52 and 54 for my 8.5 and 9.2m sails for light wind planing and upwind performance. The 54 fin was good up to about 15 knots, the 52 was good up to 18-20 knots and then I put on the stock 48 fin.
I lighter winds, I believe a 50 or 52 fin will make a difference on the futura. I weigh 79kg