Ribbons are sold on bolts by size number which indicates their width.  These numbers are standards that originally came from the millinery (hat and hat trim making) industry.  The exact measurements in inches have changed some over the years and recently have officially gotten smaller.  Below are the current standards for the # sizes.  The number of yards per roll (sometimes referred to as “put-up”) varies by manufacturer, so don’t be afraid to ask.

#1.5  5/16”
#2   7/16”
#3   9/16”
#5   7/8”
#9   1 5/16”
#40   2 ½”