Wow, that would be one serious vertical display...!
Anyway, Plasmas have a native resolution (like LCD's) so you are probably best finding that resolution and sticking to it (i.e. get mame to output all games at that one resolution).
When you feed in different resolutions, the Plasma is simply scaling or shrinking them to fit its native resolution.
Unless its HD, it will probably have 480 pixels vertical. Is it a widescreen? The manual will probably tell you the native number of pixels.