Dahlias are available in different heights, dwarf dahlias are normally 12 to 18, and the taller dahlias grow 5' to 6'.Dahlias up to 2 ft.ahlias are available in different heights, dwarf dahlias are normally 12 to 18, and the taller dahlias grow 5' to 6'. Dahlias up to 2 ft.