Rowing is easy to pick up if you already understand reading water, as just mentioned. I was a kayker who switched to rafting, and it was an easy transition once I realized I had to set up way earlier for my lines.
I have a 13'10 raft, and I can semi-comfortably pack me and a passenger and gear for one or two nights. If you are aspiring to run overnighters on bigger rivers like the Colorado in Utah, the Green, etc. 16ft would be best. If you think you are going to run Arkansas, etc. often, then 16 would work at medium or better levels, but would be a little cumbersome for those Ark day trips, etc. I would guess that inside surface area in a 16ft boat is maybe 50% more than a 13-14ft boat, and can hold a bunch more gear.
The ideal solution: 16+ footer for big water and overnights, a 13footer for day trips, and a little creeker for those R2 trips through the Black Canyon...
Happy boating, and be sure to holler at the Buzzards if you are looking for people to run rivers with.