A standard crib mattress is 27 1/4 inches x 51 5/8 and no thicker than six inches as mandated by the Consumer Product Safety Commission's crib mattress guidelines. This fits perfectly in a standard size crib and all edges of the mattress should fit flush against all internal edges of the crib. A properly fitted crib mattress ensures that babies cannot get any part of their body trapped between the mattress and crib walls.