The Min fields in the Space Inspector's Extent And Box panel define the lower X, Y, and Z bounds of a Space. The corresponding upper bounds are defined by Maximum Extent.
To change these values, unload the Space and select Edit in Extent And Box.

The Extent And Box panel for an unloaded Space, showing the editable Min and Max fields.
The minimum and maximum extent values are constrained to prevent invalid input ranges.
See Space Extent for the complete panel and recalculation workflow.