You should double click the master you want to set up, click on the Settings button, then simply select 'Import Settings From Plotter' at the top of the list, under Paper Size in your Master Layout Settings. In the next dialog, choose Architectural as your Paper, 30x42 as your Paper Size Standard, and set the Orientation as needed*. When you OK this, the settings will be shown in your Master Sheet Setup dialog. No need to do anything else, but hit OK.
*If the orientation is wrong on the screen, you need to go back through those same steps and turn it.
The beauty of this method (although it could be more intuitive) is that it will set the paper size for any sheet that uses it as a master.
Cheers,
Link.