How big/little of an opening does a boggle need to use its Dimensional Rift ability?

Question

I'm creating a giant wasp hive in the boughs of a tree in the Feywild, and it happens to have a boggle in it.

The upper sections of the hive have smaller, narrow, rigid, doorway-type openings everywhere that the boggle can use for dimension shifts.

My concern is about the bottom two sections of the hive. The doorway-type openings throughout the bottom two sections are ten or twenty feet square on those levels (so, 15'x20' or 20'x20' or whatever).

I'm concerned that these openings might be too big for the Boggle's dimensional rifts. How large/small of an opening does a boggle need to use this ability?

Answer 1

There is no established lower bound, so that's up to the GM

The boggle's Dimensional Rift trait (Volo's Guide to Monsters, p. 128) simply says:

As a bonus action, the boggle can create an invisible and immobile rift within an opening or frame it can see within 5 feet of it, provided that the space is no bigger than 10 feet on any side.

...and the description in the Boggle section (outside the statblock) says:

A boggle can create magical openings to travel short distances or to pilfer items that would otherwise be beyond its reach. To create such a rift in space, a boggle must be adjacent to a space defined by a frame, such as an open window or a doorway, a gap between the bars of a cage, or the opening between the feet of a bed and the floor.

As long as the opening is defined by a frame of some kind, there is no lower limit to the size of the opening. As such, the GM decides whether a space counts as an opening for the purposes of the feature.