When you drag to popover to the layout in layout mode it will let you resize it just as any object can be resized. If the popover is already on the layout just open the popover in layout mode, then click the popover then drag it to size.
If you first size the portal to be very large, you can set all four auto-size anchors and this will tend to make the popover adjust to fill the screen. You may want to add a close button to your popover in such cases.
I use this for an iPhone base FM GO file where I need to provide the user with a read only scrolling "reader" of what can be a lot of text for some records in the file.