They are controlled airspace classified as Class B or Class E. Class B extends from 12,501ft or the MEA (whichever is highest), up to 17,999ft ASL. Class E extends from 2,200ft AGL up to the bottom of the Class B airspace.  The lateral dimensions of the airways depend on what navaids they are based off of. They can be VORs, NDBs, or a combination of the two.

An airway based off of two VORs will have a splay angle of 4° on either side of the center line, which will intersect with the basic width boundary of the airway 50.8nm from the navaids at either end of the airway.

One VOR and one NDB will have a splay angle of 4.34° on either side of the center line, which will intersect with the basic width boundary of the airway 49.66nm from the navaids at either end of the airway.  The same goes for and airway based off two NDBs.