I was standing the first time I took a four quart enema. I was in a bathroom with multiple bags hanging from the shower stall glass and running into one common tube and into me. I had been trying to get four quarts for several weeks. The water was flowing while I was crouching down. For some reason, this night, I just knew the four quarts would go in. I crouched a little lower and all four quarts went in! (This was back around 1988.)
My long time preference is lying on the floor on my back. I can arch on my shoulders and lift with my knees (sort of a neck bridge position) and aid the water flowing in me. I can also use Lamaze cleansing breaths and massage my stomach. Also, I can watch the container empty.
These days I usually do my enemas in a bath tub, so I can rest my feet on the front edge of the tub, or on the faucet fixtures and arch really high gushing the water into me. It's passionately erotic too! It is also convenient if I plan to masturbate while taking the enema or afterward.
I find this position most comfortable for long retentions of an enema.
However, I have trouble retaining my water as I rise from my back in the tub. The water squashes around and squirts out of me.
So more recently, I've been kneeling in the tub with my ass in the air and my face on the tub bottom. I wriggle and rotate my hips a lot, which I am sure would be amusing to a partner, if I had one. I got the idea to do that from Zity.biz.
I am ambidextrous and have never found the Simms Position useful: I can't remember if I am to lay on my right side with my left leg up near my chest, or on my left side, with my right leg up near my chest. Neither position works well for me, nor in the bottom of a bath tub. (I am too tall for that.)
I have done punishment enemas lying on my stomach. This is how I got them as a boy. Laying flat on my stomach puts external pressure on my abdomen and colon and (for me) makes any enema more unpleasant. It also makes me feel very vulnerable and helpless. This is not a good position for maximum volume.