Loons are larger with longer bodies and larger, darker bills. Horned and Eared Grebes are smaller with much shorter bills that are never yellow. Red-necked Grebe is smaller without the bold black and white neck pattern. The formerly conspecific Westerns Grebe is very similar but has a greener bill. The dark crown of the Western Grebe includes the eye, whereas in the Clark's it does not. The two species have distinctive calls: Clark's give a single "kreeet" while the Western gives a doubled note. Hybridization is known go atypical birds should be studied well for exact bill color and face pattern.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.