Fishnet with one row and column and a specified cell size
or some incarnation of Xander Bakker 's
FYI In an upcoming release, you'll be able to use a new polygon tool called the Regular Polygon tool. The tool will allow you to set the number of sides for a polygon that you will create, preview of the functionality:
Here I set the number of sides to 4 to make a square. I can create the square centered on the point by snapping to it and dragging outward. If I had another feature nearby, I could also snap the corner of the polygon to it.