What dildo to choose for a harness?

There is a property called "Dildo holder diameter"(you may see it if you look under the "properties" tab).
This property will tell you how large the O ring is. You'll need a dong that's a bit smaller in diameter than the O ring size.
There is also an additional tab called "Compatible Dildos" (it's right below the Harness picture) which shows you what are the most suitable dildos for that Harness. Reply

One of the nice advantages of harnesses with o-rings is that you can switch out the rings for larger/smaller dildos. Depending on the specific harness, you might still be constrained by the cutout in the back pad of it, but usually that permanent hole will be large enough to accommodate most dildos (within reason).
Also, I'd recommend getting some metal o-rings if you do plan to use multiple sizes- rubber ones tend to stretch too easily. Reply