Which nutrients or boosters should I use to increase flower size?

You will see from the Reiziger solution labels that all of the bloom products either contain high phosphorus (P) and potassium (K) levels (the P and K in NPK) which is what you want or important micronutrients, particularly when growing in hydroponics.  These solutions are Reiziger Bloom Food A&B, Reiziger Bloom Minerals and Reiziger Bud Booster. In soils, beneficial microorganisms compete for potassium, so you need even more of it.