My rule of thumb is that the diagonal measurement of the 4:3 image on the screen should be about equal to or less than the distance from your eyeballs to the screen.
So if your cab and CP put your eyeballs 24" from the screen, a 24-25" 4:3 monitor or 16:9 32" widescreen monitor is as large as I would go.
A pedestal cabinet allows you to get some distance from a bigger monitor.
My own cab has a very narrow CP that puts you fairly close to a 16:9 27" (rotating) widescreen.
It feels about right, but on some of the vertical shmups I find myself moving my head to look at enemies on the edges of the screen.
I like to think I'd do better on those games if I could take in the playfield as a whole without moving my head to look at different parts of the screen.
(I may suck bad enough at shmups that it would not help)