XXXXXXXXL is Challenge #21. This challenge changes all floors to be the size of XL Womb but does not combine the floors of each chapter. The goal is to defeat Mom's Heart/ It Lives. Alternatively, you can defeat The Lamb by teleporting to Dark Room via a Sacrifice Room, or Delirium if the portal to The Void appears after defeating Mom.
Teleporting to the Dark Room via a Sacrifice Room is no longer possible, since the challenge does not end in the Dark Room.
Due to the floors being much bigger, it makes important rooms like Treasure Rooms and Shops take significantly longer to find, which can make the run difficult if Isaac has rather weak items. It also makes Secret and Super Secret Rooms much harder to find. Although all floors are technically XL, there is only one treasure and boss room on each floor. On the plus side, since there are more rooms, there are more opportunities to generate pickup items.
Strategy
Items and pickups that reveal the map are very crucial, allowing easy maneuvering between Special Rooms throughout floors.
Items that provide benefits based on clearing rooms, like Little C.H.A.D., will help Isaac significantly because of how many rooms he will go through in this challenge.
Teleportation cards, such as The Stars and The Hermit, are very valuable, as they allow easy access to Special Rooms. The Emperor is especially powerful, as it can be used to skip the entirety of a floor.
Avoid items that teleport Isaac at random, such as Cursed Skull, as they can significantly slow down progress by potentially teleporting him to the other side of the map. Picking up the Cursed Eye is not recommended for the same reasons.
Planetariums are very useful for this challenge. Sol can be used to find the boss room, Terra can break doors with ease, and Mercurius allows you to walk through rooms with ease. Considering you might skip an item room or two anyway in the interest of time, this can be a very effective way of completing the challenge.
Notes
Curse of the Labyrinth can still spawn as normal, combining the two floors as normal, and causing there to be two treasure rooms and boss rooms. The size is unchanged.
It is very rare to start next to a treasure room in this challenge.
Mom's Heart/ 
It Lives. Alternatively, you can defeat 
The Lamb by teleporting to 
Dark Room via a 
Sacrifice Room, or 
Delirium if the portal to 
The Void appears after defeating 
Mom.
Teleporting to the 
Little C.H.A.D., will help Isaac significantly because of how many rooms he will go through in this challenge.
Cursed Skull, as they can significantly slow down progress by potentially teleporting him to the other side of the map. Picking up the 
Cursed Eye is not recommended for the same reasons.
Sol can be used to find the boss room, 
Terra can break doors with ease, and 
Mercurius allows you to walk through rooms with ease. Considering you might skip an item room or two anyway in the interest of time, this can be a very effective way of completing the challenge.
Curse of the Labyrinth can still spawn as normal, combining the two floors as normal, and causing there to be two treasure rooms and boss rooms. The size is unchanged.
Gold Heart - Complete XXXXXXXXL (challenge #21).
Mom Unlocks 
Challenges 
The Binding of Isaac: Rebirth
