Shrikes are similar but have black masks and thicker bills. Sage Thrasher is similar to juvenile mockingbird but lacks white wing patches and has darker, more extensive spotting below. Rare Bahama Mockingbird (stray to Florida) has streaked flanks and lacks wing patches.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.