Welcome to the forum. Not quite sure what your asking. Sound like you have a winchester 1400 in 20ga. Your bore size is .586 (which is a standard bore size for 20ga.) If I'm understanding you right you want to know what size you can shoot with the new steel shot (instead of lead). 20ga is 20ga. Shot sizes are the same. Is this what you wanted to know?
Its your gun and you can do what you want but me personally I would say no to steel shot. If you don't care about scuffing and erroding your barrel then I say blaze away.
Any reason why your wanting to shoot steel shot? Most people reserve steel shot for waterfowel b/c of regulations not personal preference. Lead is more dense then steel so it take a lot more velocity to get it to punch like lead. Many people don't like that type of abuse on there shoulders for everyday shooting.