This seems a hot topic. 
Diameter: 35cm to 40cm seems pretty good for general use. In tight places 30cm or even a bit smaller if transport is a problem. On wide open spaces 50cm is good or even 55cm. 60cm was too much - the weight increases as does the drag. The latter can be eliminated by using more open material, but then the smallest ones might slip through.
Length: I prefer roughly 1.75 x diameter. That is long enough to flip reliably, but not so long that you can’t peek in to see what you have (after they’ve settled down). If you don’t do that, the 2x diameter or a bit longer should work.
Wire: Piano wire is the best. For diameters under 30cm d2.5 mm will do. The general use is d3mm and above 45cm I would go for d4mm wire. If the wire is standard steel, you’ll want to increase one step or you are likely to bend it. The reason for increasing the thickness on piano wire is not strength - it is flexibility.
Does it need to be foldable or can you transport it without? Foldables are available, but usually those are weaker material and also thicker. This adds weight and it adds quite a lot for larger diameters.
Is the ring permanently attached to the handle or removable? That is transport issue again, but with removable ring you can mix and match diameters and handle lengths for different situations. So a standard fixing is preferable if you need more than one.
Usually handle lengths follow the diameter, but not entirely. A short 1m handle with 40cm diameter works very well on mothing around a light. Normal handle length would be 1.5m to 2m and even longer ones for special purposes are possible.
Can you transport a fixed handle or does it have to be collapsible or even telescopic for multi-purpose use? I made one from telescopic walking stick and it was quite handy except that it was the open-by-twisting type and tended to open when twisted the wrong way. The current ones don’t do that.
The fixing of the ring to the handle is often critical for longevity. I wasn’t happy with a commercial ones so I overdesigned one for myself. 3D printed out of stainless steel with no fatigue point anywhere. That will never brake.
The hole at bottom gets threads and a threaded rod with strong LockTite. The ends of the ring are bent 180 degrees and tightened with a bolt. The offset between the sides adds some rigidity too.
OK - forgot to add that my comments are most fitting for catching flying things. If you want to hit bushes, you need more strength and likely thicker wires.