Here's an excerpt from the FARs.

 

(c) Width. Characters must be two-thirds as wide as they are high, 
except
the number "1", which must be one-sixth as wide as it is high, and the
letters "M" and "W" which may be as wide as they are high.
(d) Thickness. Characters must be formed by solid lines one-sixth as 
thick as the character is high.
(e) Spacing. The space between each character may not be less than one-
fourth of the character width.

 

 

I cheated a little on the spacing because it looks better.

 

Mike