These scales are aluminium and are suitable for many uses. 
 
Large Size - 22.2mm x 35.8mm with a 8.9mm hole. Recommended ring sizes for this size scale are 16swg 5/16" (AR 5.2)
 
Medium Size - 17.8mm x 27.9mm  with a 7.24mm hole. Recommended ring size is 18g SWG 1/4" ID
 
Small Size - 14.3mm x 22.2mm with a 5.6mm hole. Recommended ring sizes are 18swg 3/16" (AR 4.1)