You were unable to find a discussion, or unable to find a very specific length value?
If you click on the search button on this page, then enter "or" you get several threads, including these two, which go on at some length.
http://forum.daysailer.org/forum/viewtopic.php?t=2756
http://forum.daysailer.org/forum/viewtopic.php?t=2451
One of them starts out with a poster reporting that he has 8 1/2 ft oars he wants to use (I believe, from memory) he concluded at the end of the discussion that this was successful.
You could also send him a p.m. with a direct question.
Owners who have installed oarlocks are a bit of a minority. I suspect that nobody really is an expert at dimensioning oars here. Those who've tried certain lengths might be able to report what worked for them, but I wouldn't be sure that this would work for you. Like the poster, they might simply have tried whatever length oar came in handy.
Or you could look at what rules other people, who are more experts in rowing, use for selection of oar sizes.
http://www.shawandtenney.com/wooden-rowing-oars.htm
gives a formula that's based on a fixed leverage of 7:18, plus a 2" overlap when the oars meet.
The leverage ratio is clearly dependent on the design of the oar blade, so the value here may work best for their style. The following site discusses the factors that go into sizing an oar, without resolving that into an actual formula.
http://www.concept2.com/us/oars/selection/oarlength.asp
They give these guidelines:
* The more efficient your blade, the shorter your oars should be to keep the load reasonable.
* The slower the boat you are rowing, the shorter your oars should be.
* The narrower the spread of your oarlocks, the shorter your oars should be.
* The longer your reach, or more specifically, the greater the angle of your oar at the catch, the shorter your oar should be.
If you like a really detailed calculation to follow along, here's
http://www.woodenboat.net.nz/Boats/Oarchoice.html
He ends up with an oar length of 8 1/2 ft, for a boat that is about as long as a DS, but apparantly a bit narrower. His boat should be going faster (so your oars for a DS should be shorter) but his boat is narrower (so yours should be longer). That cancels out, more or less, and would mean that if you have the reach and strenght that he does, you could try 8 1/2.
I would simply plan on buying / borrowing one pair, and then correcting the choice when you've realized what actually works for you when trying to move a DS.