The most common dwarf hairgrass specie is Eleocharis acicularis, and it can easily get 4 or 6 inches or taller. Eleocharis parvula or parvulas is supposed to be shorter, but the difference between the two species is minimal. E. vivipara can also get that tall or taller quite easily. It really comes down to growing conditions. Amano uses acicularis both in the background and foreground. If it gets too tall, cut it.