I would recommend getting the biggest (legal) net that you feel comfortable throwing. When you're trying to catch shad at 50ft plus, you need a big net because it's going to shrink in diameter as it sinks.
As the overall net sizes go up, typically so do the mesh sizes. I'd get the smallest mesh size offered when buying a large net. I couldn't tell you what mesh size I have without looking but I want to say 1/4 inch. Smaller mesh will eliminate some of the threadfins getting stuck in the net at the surface.