e-bay length

i was told that
"In your rocket design is the electronics bay going to double as a coupler? If so, most would suggest at least one airframe diameter to slide into the body tube on each side."
but i can't get why it has to be at least one airframe diameter to slide into the body tube on each side ?

A shorter coupler may not keep the two body tube portions aligned tightly under stress? In general, I like a coupler to be that long anyway.

First, the rocking play in a coupler's fit is based on the difference between the coupler's OD and the tube's ID, which will be a distance, such as .1 mm. The angular misalignment is based on the ratio of that distance to the insertion length, so the deeper the insertion the straighter the fit. One diameter is a rule of thumb, not an absolute minimum, or a perfect optimum, or anything like that.

Second, if one end is going to be glued (not often the case with an e-bay, but quite often with other couplers) then you need a decent amount of surface area in contact for a strong joint. Again, one diameter insertion length is a decent rule of thumb, but probably overkill for this purpose.

In pictures, exaggerated for clarity: