size of the fly depends on the line wt of the rod, and what style of fishing (topwater or poppers) or nymphs for dropping down in the water column. Don't want to use a heavy bass popper/streamer on a 4wt, or a small foam topwater fly on a 8wt, very difficult to cast as the "line" is what you cast in flyfishing, not the lure or fly.

Suggest you get a wt forward (WF) floating line to start out with, learn how to cast by watching youtube videos, take a lesson or two and practice, practice and practice. GL