I can't really answer that because all adult BPs aren't the same size...Knowing the gender would help some, but all snakes grow their whole life, & even though they slow down as older adults, some just get bigger than others. Hides* aren't a big expense & I'd go bigger...don't forget you have to leave room for that moist substrate, & you can always add more substrate to make it cozier. I truly get a kick out of watching my snakes with their humid-hides...they really appear to enjoy them, & seem to be especially attracted to the smell of damp moss. *I usually use large plastic food containers with snap-off lids...they come in all sizes, even 'cake-size'- and I cut a doorway into the top. For a humid hide, you don't want the bottom open like with regular hides, as the moisture will spread thru-out your enclosure...you want it contained.