When does a word qualify as a regionalism?

What makes a usage "local"? I always thought a local/regional usage was one that was specific to a fairly restricted area--the way a milkshake might be called a "cabinet" in Rhode Island, and a "frappe" in other parts of New England. (Or the way these guys are variously called "pillbugs," "sowbugs," or "roly-polys," depending on where you live.) How large must the area of usage be, before it no longer qualifies as a "regionalism"? And can a word be called "localised"/"locally-bound" if the "locality" is the bulk of a pretty big continent?