I know this is old, but for people still looking for answers coming from google etc.:
Add a new body (temporary collision box), and drag it to near the bottom of your mesh, a little higher than where you’d want the floor to be. Save your PhysAsset and restart the engine.
The floor changes depending on the lowest collision box in the PhysAsset. There will be a gap between that collision body and the floor, so just use trial and error to get it to the height you need.
This is a long winded work around because you have to restart the engine each time, but it works. Hope this helps somebody.