Get an RO buddy from any of the aquarium chain suppliers online, its $100 and has a DI unit on it to makeclean water. I use it for my cham.and reef tank.
A full sized dubia is roughly thumbnail size give or take. I feed them to my Panther Cham Zorak at 10 months old he eats them no problem. But,they also fit the general feeding rule of space between the eyes.