Hi John, depends a bit on your smallest sailsize you are going to use. The s-type is recommended for sailsizes from 4.5 up to 6.5, but for sure you can go a bit bigger, too. The finsizes are from 28 to 38 cm. 28 will be pretty short. I would go for a 30. Best sizes are in between 32 and 36, freeslalom or slalom shapes.
Which fin sizes do you have?