In a mock Texas, small can be quite funny; Greater Tuna. Comedy by Jaston Williams, Joe Sears, and Ed Howard. Starring Messrs. Sears and Williams. Directed by Mr. Howard.

A miniaturist view of Texas? The authors must be kidding. And indeed they are. In ''Greater Tuna,'' Jaston Williams, Joe Sears, and Ed Howard are paying mock tribute to ''the third smallest town'' in the Lone Star State.

Fortunately for antic purposes, Tuna is amply supplied with caricaturable characters. Even more fortunately for the production at the downtown Circle in the Square, coauthors Sears and Williams are on hand to populate the community. Each actor plays 10 roles in addition to supplying incidental animal noises and sound effects. Besides his other impersonations, Mr. Williams specializes in juveniles (delinquent and otherwise). Along with such townsfolk as a law-and-order sheriff, Mr. Sears creates memorable portraits of two ample Tuna sisters.

''Greater Tuna'' is not so much a formally structured play as a funny-paper montage of sketches and monologues set loosely within the framework of radio station OKKK's broadcasting day. Sears and Williams serve as OKKK's Huntley and Brinkley. (One has to assume that Tuna hasn't quite caught up with Mudd and Brokaw.) Along with hayseed jokes in TV's ''Hee-Haw'' tradition, ''Greater Tuna'' takes satirical sideswipes at bigotry, book censorship, klansmanship, and the moral flaws sometimes lurking beneath the moralistic surface of heartland America.

Like station OKKK's editorializing, the social commentary is delivered off the cuff and on the wing. Under collaborator Howard's direction, teammates Sears and Williams neither miss nor overstress the points they are making. Their marvelously funny quick-change acts are played in the key of high C (for comedy). The acting remains consistent even when the material seems uneven.