Since ray and Hawk are being uncustomarily facetious, may I add that a largish mirror is normally used to enlarge a small room. The additional reflected light assists in this outcome. However, don't overwhelm the room with too large of one. Additionally, be judicious is which wall you eventually select since placing it where an afternoon sun reflects badly or someone from the outside can see everything in your room by relfection needs to be considered.
A large painting has it's own problems. If it's a predominat color, then the entire room, from paint to furniture color can be affected, regardless of wall for display. Keep in mind that when one enters a room, the painting or mirror shouldn't be the first thing that draws the eye. The room should be balanced so as to present an overall impression... in my humble opinion... Best of luck!