Best floor dimensions for adult female BP

[Maymay]

You are correct that 12 is too narrow. 48×18. would be the better option but if possible, I'd say look for a 48×24, especially since you said she's fairly active. That floorspace is readily available in several models from Animal Plastics if you want to consider pvc cages :p
Can't recommend them enough!

Thanks for the recommendation and advice! On the internet I see a lot of people keeping them in narrow tanks but it definitely seemed a little odd to me.