Probably the 3 best makes of musky nets are Frabil, Beckman and Stowmaster, but as far as I know Stowmaster is the only folding musky net, but anyone that uses them says their pretty good.
As far as size goes, it's a personal choice but the bigger the better for the size of fish your gonna be getting.
Just remember though, that if you fish alone, the bigger the net, the harder it is to use with one hand while you work the fish with the rod in the other hand.