The three shapes Instagram uses most
A square post is 1:1, at 1080 × 1080 pixels. A portrait post is 4:5, or 1080 × 1350, and fills more of the screen in the feed. Stories are 9:16, a full phone screen at 1080 × 1920. Instagram's help page says photos are shown at up to 1,080 pixels wide, so a wider photo is scaled down when you post it.
Sizing the photo yourself means you decide what stays in the frame, instead of finding out after you post.