Create a BSP box. Resize it and position it on the level. Convert the box to Blocking Volume (using the Convert Actor in its Details panel). The resulting box is a cube, which discarded the carefully chosen dimensions and rotation of the original BSP actor.
Hey
Thank you very much for your report regarding the BSP reverting back in size once it’s converted. I have commented on a bug that we have in for 4.8 regarding the conversion actually being greyed out. This bug can be referenced as: UE-16608.
If you have any further questions, please let us know. Have a wonderful day!