18" viewable sounds like a 19" tube.
But more to the point you should be considering the resolution.
My 19" CRT is currently running at 1280 x 1024, so whatever the number of inches I wouldn't want to replace it by a flat screen with fewer pixels.
(In fact when I bought a new computer recently I bought a new flat screen as well ... and then passed on the new flat screen to another family member with the old computer, keeping the old CRT monitor to use with my new computer. No doubt the day will come, as flat screens continue to be developed but CRTs probably don't much, but so far I've never seen a flat screen that is as good as a CRT.)