Dual-rank 16GB sticks fit fine in mini-ITX.

Yes, physically they fit. Dual-rank and single-rank RAM sticks are the same physical size—same PCB height, same thickness. The “rank” refers to how the memory chips are organized internally, not how big the stick is.

The only clearance issue you might hit on mini-ITX is with a chunky CPU tower cooler hanging over the DIMM slot, but that has nothing to do with whether the sticks are single- or dual-rank. If your cooler already fits standard DDR4/DDR5 sticks, dual-rank will slide in just the same.

Worry about cooler clearance, not rank.