A 27'' TV will work great for an upright cab, and even something bigger, like a 29'' will be ok. I am using a 27'' TV for mine, and it is about 24.5'' wide. I believe with a 28'' inside width, you could fit up to a 32'' without problems, but ALWAYS double check that the monitor will fit. S-Video is a must, as the picture quality is increased dramatically. In my opinion, a flat screen TV does not belong in a cab, and I believe that the regular rounded tube will give you better results anyways. Just make sure you buy a quality TV, as the picture can vary greatly from a no-brand tube and a well known company.
Hope this helps, Fred.