Miller, 25, has made nine Championship appearances for the Millers this term.
"Mickel is a tricky player with pace who can play wide or centrally," boss Keith Curle told the club website.
"He is a goal threat himself and he is also a player who can create opportunities for others.
"Ryan is a mobile, hard working, aggressive centre forward. He has a hunger and a desire to score goals and he is capable of scoring different types of goals. He can link play well and he also has an excellent work ethic."