Depends on what you want to do with the net. If you want to catch isopods, obviously you should use fine mesh. Color is for you to see the net, but you may want it to be as noticeable to the thing that you want to catch. Softer fabric is safer, but for a heavy shark, you may not have a choice. Same for size of the net, but I feel I am just talking thing that everyone knows.
Again, depends on what you do, everything is just a compromise of one way or the other.