When 1 drawing unit = 1mm:

After selecting the location of the block you selected enter "25.4" at the first prompt.

Example:

Insertion point: X scale factor <1> / Corner / XYZ: 25.4
Y scale factor (default=X): ENTER
Rotation angle <0>: ENTER
command:

When 1 drawing unit = 1m:

After selecting the location of the block you selected enter ".0254" at the first prompt.

Example:

Insertion point: X scale factor <1> / Corner / XYZ: .0254
Y scale factor (default=X): ENTER
Rotation angle <0>: ENTER
command: