The CICS screen is not fixed like what u told (24*80). The screen size is varies based on the main frame screen resolution.
so that we can scroll the screen with PF7 and PF8.
generally we are using 24*80. some body they are using 40*80. so u r not fixed for only for one screen. Ans is depending upon the system. that 's way u r declare in mpset termial=all or 3270. 3270 is 40*80.