I would start off by finding out what you can from the product specs - size, resolution, color-depth. There may be a part number on the display itself that you could use to track down additional information. This would of course require that you tear apart your Zune
From there, Google-Fu will be your friend. The display may or may not have it's own controller, and it's most likely a TAB connection (soldered to the PCB). Going with a larger display will only net you a larger display - not a higher resolution. The firmware won't support anything but it's native resolution.
The tricky bit will be wiring up a different display. If there is no controller (just a driver) you may have better luck in using a larger display. as the connectivity (pin out) will be similar enough you should be able to get it going. If the display has a controller, then it will need to be the same controller, or on that is compatible, that supports the same interface as the display in the Zune.
Not saying it can't be done, but you will have your work cut out for you.
Have you looked into the video out capability of the Zune? I hooked mine up to a TV and used the dock with remote in the past. Not as slick as what you are looking to do, but it works.