Bava Batra 101A

ורחבן ששה.

and their width is 6 handbreadths.

רבי שמעון אומר: עושה תוכה של מערה שש אמות על שמונה, ופותח לתוכה שלשה עשר כוך ⁠– ארבעה מכאן, וארבעה מכאן, ושלשה מכנגדן, ואחד מימין הפתח, ואחד מן השמאל.

R' Shimon says: He should construct the inside of each burial chamber 6 cubits wide by 8 cubits long and open up into the chamber, by digging into its walls, 13 burial niches. 4 niches should be opened up from the wall here, along the length of the chamber, and 4 from there, along the other side, and 3 niches from the wall facing the entrance, and one from the right of the entrance and another one from the left.

ועושה חצר על פי המערה שש על שש, כמלא המטה וקובריה.

The Mishnah describes the general structure of the catacomb: And he must fashion a courtyard at the entrance of the burial chamber that should be 6 cubits by 6 cubits, which is equal to the combined length of the bier of the deceased and those who bury him, to ensure adequate room for the burial to take place.

ופותח לתוכה שתי מערות ⁠– אחת מכאן ואחת מכאן. רבי שמעון אומר: ארבע לארבע רוחותיה.

And he should open up two burial chambers into the courtyard, one from here and one from there, i.e., on opposite sides of the courtyard. R' Shimon says: He should open up 4 burial chambers, one on each of the courtyard’s 4 sides.

רבן שמעון בן גמליאל אומר: הכל לפי הסלע.

Rabban Shimon ben Gamliel says: Everything is dependent on the nature of the bedrock. If the bedrock is hard and strong it will be able to accommodate more niches, which will be more closely packed together, with less bedrock between them. If the bedrock is softer, fewer and more sparsely spaced niches should be formed.