Just have one made up with the rim and hub you want. If you don't have a rim in mind, let the wheelbuilder know how much you weigh, what you're using it for, what your rear rim is, and they can pick one. Actually "building" the wheel doesn't take that long, and it's not expensive unless you use expensive parts (speaking of which, the dynamo hubs are not cheap, but I assume you knew that.) Try to find a good wheelbuilder (ask around locally if you're buying locally, many regular cyclists have had wheels built up) as opposed to just running down to the nearest bike shop.
Take a look at how the light will be mounted. On my bikes (uprights), I use a skewer mount, which works great, but that might put the light too low on a 20" wheel.
I'm not sure of the details, but I understand some of the hubs are optimized for particular wheel sizes. I understand the popularity of the Son20 is lower drag, and one reason for this is it is actually intended for 20" sized wheels.
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