Taken from AR (Army Regulation) 670–1 • 3 February 2005. I'm sure the other services have similar regulations, but I would check to be sure of the details.

29–9. Wear of miniature decorations and service medals
a. Miniature medals are replicas of regular size medals, made to a scale of one-half the size of the original. Except
for the Medal of Honor, for which there is no miniature, only miniature decorations and service medals are authorized
for wear on the mess and evening mess uniforms. Personnel will not wear full-size medals, service and training
ribbons, or U.S. and foreign unit award emblems with miniature medals. Only the dress miniature-size combat and
special skill badges are worn with miniature medals.
b. Miniature decorations and service medals are authorized for wear on the following uniforms.
(1) Male personnel. On the Army white and blue uniforms, the white and blue mess and evening mess uniforms;
and on the left lapel of formal civilian attire, when wear of Army uniforms is inappropriate or not authorized.
Miniature badges are authorized for wear on the AG shade 415 shirt. (See para 29–17b for wear of combat and special
skill badges with miniature medals; see paragraphs 29–17c and 29–18d for wear of combat and special skill badges on
the AG shade 415 shirt.)
(2) Female personnel. On the Army white and blue uniforms; the white, all-white, black, or blue mess uniforms; the
Army white, blue, or black evening mess uniforms; and on the left side of formal civilian attire when wear of Army
uniforms is inappropriate or not authorized. Miniature badges are authorized for wear on the AG shade 415 shirt. (See
para 29–17b for wear of combat and special skill badges with miniature medals; see paragraphs 29–17c and 29–18d for
wear of combat and special skill badges on the AG shade 415 shirt.)

30–6. Wear of medals on civilian clothes
Retired personnel and former members of the Army (as described above) may wear all categories of medals described
in this regulation on appropriate civilian clothing. This includes clothes designed for veteran and patriotic organizations
on Veteran’s Day, Memorial Day, and Armed Forces Day, as well as at formal occasions of ceremony and social
functions of a military nature. Personnel may wear either full-size or miniature medals. Personnel who wear medals on
civilian clothes should place the medals on the clothing in approximately the same location and in the same manner as
for the Army uniform, so they look similar to medals worn on the Army uniform.