The Instagram app accepts only vertical/portrait video format at an aspect ratio of 9:16.
The natively supported resolutions are either 504x896 or 396x704 - both are 9:16 aspect ratios.
We can stream other 9:16 resolutions like 608x1080 or 720x1280.
Regardless of the streaming resolution, the Instagram app zooms in the Live video for viewers. The zoom-in ratio is around 7.5%.
When working with streaming software like OBS or Wirecast, it's recommended to set your scene to consume less than the full-frame. See samples below.
Here's how the video looks like in the Loola Studio:
And here's how it looks like in Instagram: