The ST126 was my lightwind board for three years. The biggest sail I had on it was a 7.3 (I'm 68 Kg). I thing it can handle 8-8.2 very well but it has a quite narrow tail (~43 OFO) and 8.5 might be the upper limit. It is OK with a 43cm fin and also 36cm for 7ish sails. The board is scarying fast with 6.0 and still handles the chops well. However 5.5 seems a bit small.