The jacket pictured here is a large, so the medium jacket will probably suit most people.
There are no sample sizes available, but ask Nick if you can try on his (as seen in the pictures). Bear in mind that you will probably want it to fit over your kit.
The black jacket featured in the pictures here shows up the logo particularly well, but the choice is of course yours.
All jackets are made of soft shell material, and are both water repellent and breathable.
If you don't know what soft shell means, an explanation can be found here