Is this for catching minnows or nymphs? For nymphs your typical swimtrunk mesh may have too large of holes. For nymphs, a cheese cloth type meshing is the more appropriate size I think. Anyhow, there is no exact dimension. I would design it to fold into a vest pocket conveniently. I made one from cheese cloth about 6 in. wide and 10 inches long and it seemed to work OK.
One problem I have with my homemade one is that I attached it on the ends to two small dow rods to open like a scroll. Problem is, it sort of stretches lengthwise and become too thin and I have to keep manipulating it. Ideally, it should have some support on all sides, but then it is impossible to roll/fold up. If you come up with a good design, let me know.
