A:
the standard way to calculate proper oar size depends on the span between the oarlocks, rather than length. here's how it goes:
measure the distance between the oarlocks. divide by 2, then add 2 inches. take this number, multply by 25, divide by 7, and that's your approximate answer in inches.
for example, if a boat is 48 inches between the oarlocks, then the proper size is: 48/2=24+2=26. 26x25=650/7= approx. 93. That's 7 feet, 9 inches. thus, either 7-1/2 or 8 foot oars should work well.
performance rowers with a rowing shell may do it differently, but this formula works well for most rowing craft.