I'm not very familiar with that saw, so what I say may be off the mark, but, I'll take a shot. Run what you have to start with. That way IF it doesn't work out for you you're not out any cash. If you need to make a change after a few trial runs, I would go with a 28 inch sprocket nose and 3/8 chain in what ever pitch you can get easily in your area. I have given up on the "milling" chain, but, that's just me. I like 3/8 chain and for my normal use chains I run .050 gauge. For my long milling chains I use .063 gauge. So I would say 3/8 .050 would work for milling with that saw on a 28 inch bar. Bar length will depend on your mill. Check out what the Granberg folks say about maximum cut width and needed bar length for a given mill rail length. For instance, a MK-III with 24 inch rails will give you a maximum cut width of 20 inches but that requires a 28 inch bar.
My GB double ended mill eats up about 10-12 inches with a double powerhead setup but if I use a single powerhead and setup everything right, I only lose about 6 inches of bar length.
Scott