With 6 to 8 inch fish you should be able to go up to 3/4 inch mesh. I get largemouths that size every year and I use 3/4 inch mesh. The larger the mesh the better the circulation which is what you want.
In my pond I have to brush off algae accumulation one a week in summer or the mesh will clog up and cause problems. It depends on how fertile the pond is. You may not have that problem at all.
Here are some links on cage culture including hybrid striped bass in cages:
http://aquanic.org/beginer/systems/cages.htm
