I have used the same sice as we use fore the L 046, internal 32x22x140 m/m i have also made with 40x30x160 m/m, but they have chosen the smaller one, and they have given us some fry in these
Sorry i`m bad. I ment mm. As i sad before my L 260 could chose between both and whent for the smallest, but they are not more then 10 cm long. But i did have a 12 cm ancistrus sp witch also tok the smaller. But i can only speake for the fishes i have i my own tanks.
I would think that giving the fish a choice of some different size/shape caves is always a good idea. If there's no choice, they may not feel happy about the cave and choose to not spawn...
The cave should be sized to just about be able to hold both the male and the female...