I just completed a 2 day river rescue class sponsored by Summit Kayak School here in Colorado, and taught by Miah Wheeler (great instructor).
The length of rope in a throw bag was shown to be a significant issue during training.
I happened to have bought a 70' foot bag before the class, and I'm glad I did. You can do a lot with that extra 20'.
Also, the Salamander 70 footer "Little Big Mouth" w/ Spectra is very compact and easy to throw.
Look for an extensive journal entry with photos and videos from the 2 day river rescue course online this month.
Also, I took a 3 day IRIA Swiftwater Rescue Certification course with Colorado Mountain College in April. Photos and videos will also accompany that journal entry.
Both courses were helpful to me, but the SKS 2 day river rescue course with Miah was much more beneficial to me as a kayaker.
My advice is to buy a rope longer than 50', and then take a river rescue course as soon as possible.
for my insanely thorough Learning to Kayak Photo/Video Journal.