I agree with MsLisa. You DO NOT want massive enemas in preparation for anal intercourse. I like to do progressively SMALLER enemas as preparation.
Depending on the degree of penetration (are you using your penis or an 18" long double dong with full penetration and for how much time--5 minutes or an hour) you will need to make adjustments, but for most activities up to the 18" dong for an hour, I start with a maximum of 1 1/2 quarts, then one quart, then a pint, then a quick rectal rinse.
Wait between enemas for all the feces and water from the preceding one to be expelled. The reason for starting large and going smaller is to loosen everything and expel it, then the each succeeding smaller one cleans out the remnants of the one before.
You DO NOT want to go to bigger enemas because that will only loosen up things higher up and will keep coming down while you are playing. Go from larger to smaller.
It also helps NOT TO EAT for a good few hours prior to heavy anal play.
If you are going to have HOURS of very deep insertions (24-36" with heavy fat tubes) or deep fisting (to the elbows), you may want to do a thorough cleanout in the shower in advance with a shower shot or something similar so can you do many repeated enemas, filling and expelling---maybe a dozen fill and empties in say half an hour to an hour.
This regimen works well for both my husband and myself.