This is one of those questions that has alot to do with personal preference.

longer oars mean more power on the stroke but they stick out there farther and will be quite a bit heavier to lift from the water on the return stroke. Shorter oars less power per stroke, but lighter to lift from the water on the return stroke...

It's all about leverage and the physical location of the oarlock/pivot point on the oar.

smaller narrower boats can use a shorter oar because the oarlocks are closer together. wider boats longer oars...

I cannot give you a rule of thumb to use, I would borrow a couple of different lengths and try them out in the lake to see which you like best...

I bought 9'ers for my cataraft and wish I would've tried 8' or 8.5'