I use the woodfired pottery ancistrus caves from rosenthal, they are about 2.5" wide which is perfect for the male and female to fit in together. While the BN body is not so wide, you have to allow room for the fins, mine have very long pec and dorsal fins, as well as allow for them to spawn together in there. It will not work if there is not room for two fish to move around.
I am using his #4 style, but in the wood fired finish which looks nicer (darker) in the tank. Just started using them. Was using his cone shaped ancistrus caves for a couple of years until about 2 months ago when one of my females got her head lodged all the way in the tip of the cone and couldn't get out. She'd been in there a couple of hours so I thought I'd lost her. Much damage to her pec. fins and very shaken, but all back to normal now. These #4 caves are longer and wider, just not as tall. But the length is very nice and the males no longer stick out the end. So much room at the back of these caves that my one male just allowed 4 different females in to spawn with him and when looking in you could see 4 separate egg batches. I'm really liking the extra room in these caves. I think they run $10.
He backs up sometimes so that the ends of his tail fins are sticking out but that's a non-issue. The males are most of the time head first in the cave while fanning.
I can only tell you that you don't want it so big that the male feels threatened by other tank occupants. I'm sure there was research done on what ancistrus and other plecos prefer for caves before rosenthal and others starting making pleco caves. And these are what rosenthal uses himself with his ancistrus. My ancistrus have no issues with these caves whatsoever.