Rush creates its sequence settings based on the first video clip that is imported into your project. Is it possible that the first clip you selected/imported was 720x1280? Unfortunately, as of today, there's no way to increase the size of your export settings in a vertical view.
You could (not sure if it would be worth it) export your project as 720x1280. Create a new project in Rush using a 1080x1920 and import your 720x1280 file you exported, increase the size of the video to fit your monitor, and then export as 1080x1920.
You will lose quality by upresing video. If you really want 1080p, you can see how it looks.