The rain fly appears to be 1 sq ft. It should cover most of the body. It will not help in anything more than a sprinkle
The rain fly covers the open vent at the top. The rest of the tent is water resistant. So if you cover the vent at the top, rain can’t come in the top and will run off the sides, but your tent will still “breathe” which is ideal.
A 1 foot fly should be plenty for me with some left over.