What unicellular eukaryotes reproduce sexually?
This addresses your original question of "simplest" versus smallest. I'm not sure of sizes.

Investigate diplomonads: https://en.wikipedia.org/wiki/Diplomonad Most notable among them is Giardia: https://en.wikipedia.org/wiki/Giardia

These critters are primitive enough to lack Golgi and their version of "mitochondria" are substantially different enough (not synthesizing ATP) to be called by another name, mitosomes. Most have two nucleii and were not even thought to reproduce sexually  until relatively recently