16" wheels on a 27" road bike with 3.5" crank arms just barely clear the ground if you don't lean. have one. I'll try to add photos this weekend. 3.5" crank arms are not easy to get.
had a 20" on a 27" road bike that had a bit more pedal clearance.
i've built a few 24" wheels on 26" bikes. those ride pretty good. You can usually fit fatter tires when you downsize. Swaping out crank lengths can give you a little more ground clearance. This is one I built a couple years ago. Ground off the canti brake mounts and cable stops/guides and the 2 chainrings that were riveted on. Coaster brake rear hub.
