Over the last several years, neoprene/hypalon material has been getting a lot of competition from PVC plastic. Also, in the last year or so, a new generation of material (eg. "Lexatron" from Whitewater Manufacturing) has entered the market. Another inovation is the "cataraft", which is raft formed by putting a frame across two "outboard" tubes. Rafts are evolving rapidly (sorry).
Most human-powered rafts range from 10 to 18 feet in length, with most boats being 12 to 16. 10 foot boats are used by people who want to get very wet, or by pairs of paddlers, often on water too technical for larger boats. 18+ foot boats are used as gear boats on Grand Canyon style trips. Oars for common sized rafts are generally 9-10 feet. Rafts are classified by the number of paddlers, usually: e.g. "R-4", "R-8".