Design/Options (D, O). You can set the sheet size manually, but you'll probably want to create your own templates including title block and default parameters.
Select a different template using Design / General Templates / [whatever]. You need to specify the path to your templates, and then they will show up in this menu.
Incidentally, the "standard" sheet sizes are nonsense. 'Letter' is not 8.5 x 11". Better to set it custom.
Tim