As cmgap explained, Rush allows you to manually set the aspect ratio of the sequence via the button below, which sits below the monitor on desktop and iPad; on iPhone, you'll find it on the bottom bar.

But aspect ratio is very different from frame size (or "resolution"). Rush supports three resolutions: 720, 1080, and 4K (2160). If any video clips are among the initial media added to a sequence, then Rush automatically matches both the aspect ratio and resolution of the first video clip. If no video clips are among the initial media, then Rush defaults to landscape at 1080.
Photos that are larger than resolution of the sequence are automatically scaled down to fit in the frame.
If you're working chiefly with digital photographs and you want the highest resolution available in Rush, then I recommend keeping two 4K video clips handy (one landscape, the other portrait). Use one of them when you create the sequence. That will make the sequence 4K. You can then delete the video clip and add your photos.