Entirely red head of adult should easily separate it from all but Red-breasted Sapsucker (which does not overlap in range, and has paterned back, yellow belly, and a different wing pattern). Note the Red-headed Woodpecker's different position of the white wing patch (especially in flight), white belly, and larger size. It may be confused with the largely dissimilar Red-bellied Woodpecker at times, but attention to the gray face and throat, and barred back and wings of the Red-bellied should eliminate any confusion.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.