About Oar Length
When it comes to oars, there is no hard and fast rule. Some rowers install the oar handles to lap 1", most rowers prefer a small clearance. To answer your question using extremes: if the oars were 6' long, and mounted so that the handles just touch, they would have to be at an extreme angle in order for the oars to touch the water, requiring your hands to be relatively high. If they were very long, the rowing angle would be more comfortable, but the oars would probably be unwieldly. Long oars also require that the handle end be lowered more for the oars to clear the water on the back stroke. This can be a problem if the seat height is close to the oar lock height.
R. D. Culler in his book "Boats, Oars, and Rowing" (out of print) gives a rule of thumb that the oar length should be twice the beam plus 6", with the caveat that length will vary with type of boat, rowing style, etc. A formula that I like, because it seems to have some logic to it, comes from the book "Sail and Oar" by John Leather (out of print). Figuring the ideal oar lock position as 7/25's from inboard end, the oar length is half the beam, divided by 7, multiplied by 25. Neither formula takes into account, the height of the sides (the higher the sides, the longer the oar needed). If you figure oar length using both methods above you will come up with a big difference... I said there is no hard and fast rule.