5.0.6 - 5.0.8 you can use the methods listed in
Bug 28471 - option to disable redirect to lite client for 800x600 screens to workaround the redirect implemented by
Bug 27521 - Redirect to lite client for 800x600 screens
5.0.9+ use:
zmprov mc COSName zimbraPrefAdvancedClientEnforceMinDisplay FALSE
or
zmprov ma
user@domain.com zimbraPrefAdvancedClientEnforceMinDisplay FALSE