The measurements (approximate) I got were:
Outer diameter - 17mm
Inner diameter - 14mm
Thickness - 1.5mm (make sure they're the same thickness as the gasket)
The square section o-rings fit snugly inside the oil returnway holes in the gasket. Bear in mind that if you use round section o-rings, they're unlikely to fit quite so closely and will squish differently to square section types. Also, there's some talk on the US forum about the material these rings should be made of - the general consensus seems to be viton because it stays elastic under heat and pressure.
One other thing to note with earlier bikes - the 2 o-rings that sit around the oil feed holes between the cylinders and cylinder head are also square sectioned. Unlike the others, these larger rings are still available from Honda - part number for those is 91303-035-000. These are thicker than the other 8 (maybe 2mm instead of 1.5mm) because they have recesses to sit in.
----------------------------------
I had the foresight to install one of the Gordon frame kits (see the US site for details) and so I can strip the top end in the frame. Doing the strip and rebuild properly (from complete bike down and back up again) took me around 6 hours. If you've got to take the engine out, figure another hour or two into the equation.