Rubber bands come in a variety of sizes. A comprehensive chart is here.
rubber bands found in my kitchen as well as purchased size 64 bands. from left to right:size 33 (3.5" x 1/8" thick), generic size 64 (3.5" x 1/4" thick) (480/lb), latex free #64 (400/lb), 80% rubber size 64 (325/lb), size 82 (2.5" x 1/2"), size 94 (3.5"x 5/8").
We've found for making rubber band harnesses, size 64 is both extremely common and just the right size. A size 82 can replace a doubled size 64 in the harness, but 84's are generally only found around broccoli, not at the office supply store.
Size numbers don't give all the size info, as they don't account for thickness. Thickness can be judged based on the number of bands in a given pound of rubber bands. For instance, Alliance sells three types of size 64 bands-- the cheapest has 425 bands per pound, the mid range 320, and the high end 275 per pound Office Max and Staples sell size 64 rubber bands with 380 per pound
Rubber bands are generally made out of natural rubber mixed with a certain percentage of synthetic rubber, usually EPDM (ethylene propylene diene monomer), which is more durable through a range of temperatures, as well as resistant to UV and oxidization. Synthetic rubber bands aren't significantly stronger than natural rubber, but they do last longer. Less expensive rubber bands tend to have a higher natural rubber content. Pure EPDM bands are sold as 100% latex free, for those with latex allergies. I have not found black EPDM size 64 industrial-grade EPDM bands for sale, but those would be ideal for harnesses.
size 64 rubber bands were tested with a 15lb weight attached. Wear occurs mainly where rubber rubs against rubber.
While all tested rubber bands withstood shock loading of 15lbs, all showed damage where the rubber band stretches against itself. While damaged bands were able to hold the weight, repeated shock loading lead to breakage. In the case of thin, (480/lb) rubber bands, they survived 1 shock loading and broke on every subsequent attempt. Alliance 100% latex free rubber bands (400/lb) survived 1-2 shock loadings before breaking. Office max "extra long extra wide" bands (325/lb) survived 2-4 shock loadings.
Rubber bands were also tested cold, after placement in a freezer. No performance difference was noted. However, I have noticed rubber bands weakening after exposure to temperatures below freezing.