I have a tin of buttons that belonged to my great-grandmother. I've wanted to use them but was unsure of the size of each. I don't mind mixing matching and guessing but I wanted to sort them by size and didn't know what size to label each group.
I found this chart for button sizing at www.karimeaway.com:
I printed it at 100% and labeled the sizes according to the chart. Now if a pattern calls for a 9/16" button I'll have a clearer idea which ones to use.