Hi Bob!
The Ortlieb hooks alone are a nominal 16mm. The spacers come in two sizes, one set for racks of ~8mm in diameter, the other for racks of ~10mm. I say "~" because the hooks are about 0.1mm to 0.23mm larger than the intended rack diameter; there's a little "wiggle-room".
Only caution I can offer is to say the success of rack-wrapping depends on the wall thickness of the tubing used and the friction coefficient. I had terrible luck using EDPM fuel-line rubber as it was too sticky and didn't accept or release the hooks very well. The cross-hatch-reinforced clear vinyl-based stuff Matt is using seems to work the best of anything so far. It also looks pretty snazzy with the little cross-hatchings looking kinda high-techy.
Going "wrapped and adapter-free" is nice over the long haul, 'cos with wear and friction, the adapters can become loose in the hooks and can begin to fall out. People have tried all sorts of solutions at that point, from gluing the adapters in with SeamGrip to tying them in with dental floss...none of which seems to work too well, but are viable shorter-term solutions while on a very long or remote tour. Leave the adapters out and wrap the racks, and all those potential problems go away *and* the rack stays pretty and unabraded.
Despite all the advantages I uh, haven't done mine yet. 'Need to get with it.
All the best,
Dan. (...whose usual motto is *not* "Don't do today what you can put off till tomorrow")