That width is INTERIOR width.
If you built it to 25", you have 25" for a monitor.
I remember reading a thread where someone got a 25 in monitor into a centipede with a custom frame, so I'm pretty sure you can fit a 27in monitor in there if you customize it.
I built mine to 27in on the top.... I can just fit a 27in monitor, but it's also got a large frame on it.
--NipsMG