You are stuck with what Excel will accept due to Excel's method of setting width
and height.

Row heights are measured in points or pixels.  There are 72 points to an inch
and "maybe" 96 pixels to the inch.

The number that appears in the Standard column width box is the average number
of digits 0-9 of the standard font that fit in a cell.

When dragging the column width you will see two numbers.........one is width as
above, the other is pixels.

For an interesting and enlightening discussion on this subject see

http://snipurl.com/dzz8

If you want to use VBA to set height and width in mm.

Ole Erlandson has code for setting row and column dimensions which will not be
affected by resolution.

http://www.erlandsendata.no/english/index.php?d=envbawssetrowcol

Gord Dibben Excel MVP