The smallest chainring that will fit on a Quarq SRAM XX1 Eagle DUB Power Meter is a 32T round chainring or a 34T oval. This is because if the chainring is smaller than this, the diameter of the chainring would be smaller than that of the power meter, and you would not be able to mount the chain on the chainring properly.
However, there are a couple of exceptions to this:
Round – 30T
The Wolf Tooth Components 1x 104 BCD MTB Chainring in a 30T will fit due to a small cut-out on the inside of the ring.
Oval – 32T
Wolf Tooth Components 1x 104 BCD Elliptical MTB Chainring in a 32T will fit – just barely!