I have a 1-and-a-half liter bottle with changeable screw on tops and tubes, but I very rarely fill it. I just use about 5 to 6 ounces. If you put too much water in you are going to expel it pretty quickly which is good for a cleansing enema. If, however, you are medicating yourself or introducing live organisms then you will want to retain the water and to do this you can only use a little. It's amazing how far up the colon a little water travels--I can hear it gurgling all the way up. Also lifting your legs up against the wall or putting them on a chair and tilting your pelvis allows gravity to do some of the work. Good luck with it.