1 mL should be fine. Don't shoot the ba water directly on the wafer. Angle it against the glass and slowly fill the vial, letting the GH wafer soak it up. Don't shake it, draw it up too quickly or shoot it too quickly either! I pretty much destroyed 10-20 iu before reading up on those aspects!
Just fill with 1mL and then every 10 lines (or every 10 unit mark) on the insulin syringe is 1 iu.