There are 3 sizes of circular hole available:
.125" (1/8") - Suitable for use with many common tagging guns, fine string or twine, very thin ribbon, etc
.25" (1/4") - Suitable for use with fine to medium weight string or twine, thin to medium width ribbon, etc
.3625" (3/8") - Suitable for use with thicker string or twine, thick ribbon, thin rope, etc
While ribbon can be used with round holes, it can end up being folded/curled and not lay flat. If a flat ribbon is desired, or for applications where a round hole is not suitable, "badge" style holes are available in the following sizes/configurations:
.625" wide by .125" Tall, Centre Top
.625" wide by .125" Tall, Centre Top and Bottom...