Any big streamer fly - most of mine are in the 4-8 inch range. I'd recommend an articulated style with two sturdy hooks. Here's an example of a typical fly from my muskie box:
I use Saltwater hooks. Honestly, love the bucktail and flashabou.
I don't think it is possible to tie a muskie fly too large either.
****They love eating smallmouth bass. So either tie something that looks like a smallie, or catch a smallie and let him get into a deep pool and hold on! That's your call.
I'm new to the site but am psyched to find some people talkin Musky fishin on flies!!
I've only been chasin Musky for a few years now and am just over a year into it really focusing on the flyrod. Have only a couple of smaller ones in the net on the fly but have lost quite a few nice fish.
Anyhow, I have been buyin flies here and there for a couple of years to find some patterns that work and then plan to get back into tying this Winter. I came across a surface fly called the "Poledancer" over the winter. I picked one up and have only tried it 2x so far. The action is incredible and I missed 3 fish on it. One being a low-mid 40's fish. Scared the crap out of me!! LOL
Small stream big stream it dont matter these things are built and wired for crushing anything hey want. You cant get something too big well with in the realm of castable with a fly rod .