the average arcade cabinet will take the weight of pretty much any tv you can fit into it as they are built to withstand the weight of a standard arcade monitor + people hitting them when they lose (we've all done it), as for whether its too big or too small that is all down to personal preference and how big YOUR cabinet is. the basic slim jamma cabinets tend to use 20" monitors, larger machines (mortal kombat, SFII etc) tend to use 25 / 26" monitors.
if you are planning on using a standard TV and your Xbox I would suggest that you find yourself one of the older FST toshiba TVs,
they come in sizes from 17" up to 26"
They do not have a perfectly flat screen (flat screens never look right on arcade cabinets)
they have a slim border to the top & sides of the screen (making them ideal for dropping into a cabinet)
they also have plenty of inputs (2x scart + 1 SVideo) allowing other things to be connected aswell (ps2 / dvd etc)
both scarts support rgb input and they dont suffer with ghosting from the tuner side coming through on the scarts (many modern / cheap tvs ghost from the tuner through to the scarts).