It's Easy, Being Green...
Here's a quick link to request a Rosco "swatch book," which contains a single, shoe-mount-flash-sized sample of just about every gel Rosco makes. (It's about 2 inches thick!)
If you are going to use them on your strobes, you simply need to extend the size a little by sandwiching some tape onto the edges. The gel itself will cover your shoe-mount strobe face just fine, but it has to have some extra length created to have space to fasten it with the velcro technique discussed on the "how-to gel" page.
This'll get you started even if you are
New to the gel thing? Get more info on this how-to page, from the Lighting 101 section.