I agree with the others, but if you still want your own, here's a couple of suggestions along the lines of what you are asking.
As for radius... how big a net do you feel comfortable throwing? I've seen guys with 10' radius nets they couldn't throw and a guy making 'pancakes' with a 3' net will out fish that guy in a hurry. My last net after a lot of practice was a 6' radius and that was doable. There are some great instruction vids on you tube, this one has some great 'scenery' and it's actually a very good instruction video. She's using an 8' radius net. She actually covers some of the size questions as well.
https://www.youtube.com/watch?v=w6swnJXwEws
In FL they are throwing their nets over sand and grass and that does matter. if you're throwing a net over rocks or oyster you want a smaller net and you don't want it to sink at all. it's going to get holes in it if you are doing that anyway. There are plenty areas to cast net mullet and mud minnows on clay or mud shorelines and small cuts in the marsh, same for surf, plenty of areas of sand to throw a net.
If I was getting a net made I'd think about getting smaller mesh size. I hated it when 3 inch mullet got stuck in the net head first (gill net style). So you might ask him about that. As for lead per foot you'll have to find out from others. Might search some florida boards that deal more with castnets.