You won't fit anything larger than a 25" in those things. That 23 3/4 is something of a standard inner width. Even just the tube on any larger monitor would be too wide to fit. In other words, a 27" will never fit.
A 25" with mounts should fit no problem, though.
Do you have a particular monitor in mind? You'll be able to fit a bigger monitor in there if you go with an arcade monitor. A TV has all that extra plastic that just gets in the way. Plus it has speakers and stuff you'll never use. Lots of people take the tv route because it's admittedly easier, but I think most of those people are usually building their own so they have appropriate shelves and stuff.