The average sized male bristlenose pleco will prefer the 1.25" and 1.5" openings. They like a tight fit. Large, older males and bigger species will prefer the larger 2.25" openings.
"D" Caves are good in bare-bottom tanks, since they do not roll.
Split Caves allow you to check for, and remove eggs. The "Split" caves allow you to easily remove egg clutches and hatch them artificially. If you have other fish in the tank besides plecos, they will eat the pleco fry when they emerge from the cave. Therefore, raising them artificially is about the only other option.
Square Caves don't roll on bare-bottom tanks and some plecos seem to prefer the shape.
Many people prefer to use the Round Caves when breeding plecos in tanks with gravel.
Apisto Breeding Hut measures 2.5" in diameter and is 1.25" high. These will conveniently stack on the top of the larger Cichlid Hut. Can be stacked with the Cichlid Hut.
Cichlid Breeding Hut measures 4" in diameter and is 1.75" high. They are also readily used by bristlenose.
Large Breeding Hut measures 4.75" in diameter and is 3" high. You can conveniently stack the smaller huts on top of this large hut. Although bristlenose will breed in this, it makes a great community cave where several can hang out. If you want the oblong hole to be higher, just flip it upside down. It works both ways.