Use the Geometry Data palette of the Editions main tab to modify dimensions of the selected new master page.

If the palette is not displayed, on the Window menu, point to Palettes and then click Geometry Data. The palette appears in the right sidebar. If the sidebar is not expanded, click the expander button  close to the top right corner of the main window.

Since the version 2.1, the measurement units of this dialog are read from the preferences on the Editions main tab in Fred.

Since the version 2.1, the Modules measurement unit preference on the Editions main tab is used in the Modules height and gutter fields of the Geometry data dialog box and palette. It allows you, for example, to use mm for page dimensions and pt for module height.