If you want to upload your full portrait creation to Instagram without cropping to a square, you can use the handy full-size button when uploading your photo to automatically adjust your portrait (or landscape photo) to the maximum aspect ratio available.
Just tap the expand arrows in the bottom left corner of your photo to fit the whole picture on Instagram!
Speaking of detail, the same rules about photo compression and enlarging apply here.
To get the best resolution for Instagram, size your portrait photo around 1080px by 1350px. That way, automatic compression won’t affect the quality of your vertical shot!