You canfit your whole photo on Instagram posts using resizing and editing app:
By tapping the expand arrows in the left corner of the post preview, you may prevent Instagram from cropping your 4:5 photographs to square. The post size will change to 4:5. You can use a third-party application like Kapwing to resize the picture if your image is longer than 4:5. Save your image after centering it on a 4:5 canvas.