29" (or 27" to you US guys) 4:3 15Khz CRT

That's what I use and whats used for the majority of Jap shmups.
However, if you only have the choice between an LCD or 31Khz CRT monitor I guess it depends on what your cab is going to be like. If its a slim cab then you will be looking at an LCD. If its a 'fat' cab then you could choose either.
You should be able to pick up a 21" CRT cheaper (read for free/pence) than an LCD but in this instance you may be able to get a larger LCD than a CRT.
You want to be running your games in 4:3. If you were making a Hori cab you would really be limited to a (very expensive) 21" 4:3 LCD. You could get a bigger widescreen LCD and just display the game in the 4:3 section in the middle but you would end up with a very large bezel either side of the screen and it would make your cab wider than it wants to be for the displayed image size.
This should give you an idea of what I mean:

However as you are planning a vert cab you should be able to pick up a larger LCD, mount it vertically and mask of the top and bottom of the screen as there will be much more room to house the top and bottom of the screen in.
Looking at [
this] page a 32" 16:9 LCD would give you a 26" 4:3 image.