According to this CVG story, all XBLA games must run on a 14" TV.
"Microsoft has extremely strict technical certification requirements that you have to get right. One of the biggest challenges is that your game has got to be playable on a 14-inch CRT television," said Introversion boss Mark Morris, currently working on the Arcade version of Darwinia.
"So you have to do all this visibility work in case someone is playing it on a composite cable through a TV, to make sure they can still read all of the text and tell exactly what's going on."
More XBLA technical restriction news and it happens. Don't you dare touch that dial.