I'll sit down with a smaller group. I have a class of 3 this year (plus anywhere from 0-5 online, depending on the day). Standing in front of a small group feels awkward, so I usually sit. I can see all of the kids from my seat, they can all see me, it works. With a bigger group, especially if they're big enough to have more than one row, I'm almost always standing. I don't know exactly what number would be the line.
This year adds complications. Because I have to be on camera, I stay in one place for most of the class whether sitting or standing. I'm also mostly avoiding close contact with anyone. Sometimes if they're just not getting it, I reach out physically correct hand position, but otherwise I'm explaining and demonstrating from the front.