Ok, I looked it up.
Chrono Trigger runs in standard SNES resolution, which is 256x224. Don't believe me? Look at this:
http://www.geocities.com/aaronkelley.geo/chronotrigger.gif" TARGET=_blank>http://www.geocities.com/aaronkelley.geo/chronotrigger.gif
That screenshot contains *ALL* of the data sent from the SNES to your television. In ZSNES, you can up the res of the games with filters like scanlines, 2xSaI, SuperEagle mode, etc. but in reality, this is all the detail that is there. (Chrono Trigger actually has that black bar along the bottom for a lot of the game...)
512x320: This resolution is for games that take advantage of the Super Nintendo's "High-Res" mode. The only games I know of that do this are the Secret of xxx series (Secret of Mana, Secret of Evermore, Sieken Destentsu 3 [spelling?], etc.). Oh yeah, the credits in Lufia II are high-res, 512x320.
ZSnes doesn't work? Try Snes9x... I like it better anyway.
All of the resolutions supported by the SNES are: 256x224, 256x239, 512x224, 512x239, 512x448 and 512x478. Most games run 256x224, I've seen ONE that goes 256x239, and rarely do they go higher than that.