The Norther Parula is rather small, even for a warbler. The male Northern Parula is distinctive with its breast patch. No other warbler has this feature. Females have a yellow throat and breast, wing bars, small size, and broken eye ring to identify them. Yellowthroats and Mourning Warblers are most similar in face and underpart pattern but lack wing bars. The very local (South Texas) Tropical Parula is similar in size but the male lacks the dark portion of the breast patch and has more black on the face while the other plumages lack the broken eye ring and have more yellow on the underparts.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.