Are professionals using smaller and smaller sets and boards in the top tournaments?

It certainly makes sense.  Most of us are comfortable playing online where the entire board fits in our visual field.  After playing online, my tournament mat seems enormous; it feels like I can't see the position properly.

What are the smallest pieces and board that are allowable in US tournaments?

The board size not only depends on the visual. The ability to touch the piece we want to touch. If it is smaller, my hand may touch king instead of queen and oh my opponent wouldn't leave that.