Hello Sarah,

First, Instagram compresses down every photo and video to approximately 640 px width. Instagram is mainly dealing with aspect ratios (1:1, 4:5, 9:16, etc) and the height and width of whatever you upload. The dpi/ppi is not something you need to focus on. The only reason I included it at all is that it confuses a lot of people new to Photoshop and it’s a setting you have to select.

Keep it simple:

I create all of my thumbnails/images in Photoshop:

1080x1080 (square) at 72ppi
1920 x 1080 (wide) at 72ppi
1080 x1920 (vertical) at 72ppi

Let me know how else I can help on whatever you need.

Cheers,
Mike