I've even gotten by numerous times using a square 10x10 tarp with an 11' hammock. Tarp pitched as an A-frame, not a diamond. Some posts I have at home are about 10'9" apart, so an 11' tarp is no-go.

SRL on a 11' hammock is ~110" and the square tarp RL is 120". Tarp pitched basically right on top of the hammock SRL. Theoretical 5" overhang of the tarp beyond the hammock on both ends, but in practice, maybe a smidge less. CL's of hammock stick out beyond the tarp, so driplines are essential.

I've not been through a monsoon with this setup, but have weathered a few fairly heavy, all-night storms and stayed totally dry. Sideways rain coming in at the ends would of course be the most likely failure mode, but pitched carefully and very low to the hammock, minimal tarps can keep you dry.