Hi Michael,
Unless they changed this in the 6.x series, Zimbra requires at least 1024x768 by default to allow the advanced version. It will not let you go into advanced with 800x600 without a zmprov command:
zmprov ma
user@domain.com zimbraPrefAdvancedClientEnforceMinDisplay FALSE
There is probably a way to set this for an entire COS but we only had to do it for two people with eyesight issues who didn't want a higher resolution than 800x600