I would use a short nozzle and give a half pint, let her hold that a few minutes, let her expel, then one pint, have her expel, two pints, let her expel, then four pints and let her expel. YMMV, but get her up to about four pints (2 quarts).
IF you have a double balloon rectal catheter, and she is open to it, you can use it to help her retain the enemas after you have cleaned out her rectum with at least a one pint enema. DO NOT present her with the ginormous 750cc double balloon rectal catheter it will probably turn her off. If you have a small double balloon rectal catheter, like the 250 cc or possibly the 500 cc, you can try those, but not if she has any objection to using them.
Give them slowly with her relaxing flat on her back and while giving gentle abdominal massage which can become more vigorous if she is comfortable with it. (You could massage other areas too 😁 😁 😁 , if she is recpetive to it, but keep in mind you are doing this enema series for medical reasons.) That will get just about anybody flowing again. You want several gentle enemas of increasing size. You do not want her feeling like she is being inflated like a balloon which will make her uncomfortable.
DO NOT USE KNEE CHEST WHICH IS UNCOMFORTABLE AND NOT A RELAXING POSITION TO HOLD. YOU WOULDN'T GET IN THAT POSITION IF YOU JUST WANTED TO RELAX WOULD YOU? ??? Just have her lay back and relax. I hope I made my point.
With the exception of the first little enema, IT IS NOT NECESSARY TO DO A RETENTION OF MORE THAN 2-3 MINUTES. YOU WANT WATER IN AND FECES OUT.