Ok, this has got to help someone out, I know it would help me!

Underside dimensions

Side, between wheels 51.5
Front, between wheel and center support foot 32.5
Height under bed on carpet (low) 6.75

I swapped this out so I can have more storage room under my bed and it definitely made a difference. Several other bed frames have less accessibility, but this is the best one that I found.

I have it set for a King size.

Hope this helps!