Depends on the river
I would say there is a range of values that satisfy the length for a 14 foot raft, anywhere from 8 feet (which would only be good in tighter canyons and on light boats) to 11 feet ( which would work best on big water and a loaded down 14 footer, and I mean loaded down, 11 feet would be huge on a 14 foot boat). On smaller, tighter rivers 9 foot would be good.
I really like having the purchase power of 10 footers.
Ultimately, it has less to do with oar length, and more to do with oar control.