The Display Size option (which is explained in the Knowledgebase article "Compatibility with Different Display Sizes") is a menu option in the StyleTap Application Launcher program. After you change it, the new Display Size setting applies to all programs that are run by StyleTap Platform.
On your handheld, run the StyleTap Application Launcher by tapping the "StyleTap Launcher" icon in the Windows Programs folder. (Or, if StyleTap is already running, tap on the "S" icon in the toolbar at the bottom of the screen.)
In StyleTap Launcher, tap on the title bar to display the menu bar, then select "Display Size" from the "Options" menu. The Change Display Size dialog and help screen explain how the option works.