4:5 — which is 1440×1800 pixels if you want it sharp, and 1080×1350 at the very least.
Free, and it exports at 1440×1800 without asking.
Tallest the feed allows, so it fills the most screen. The safe default.
Use it for Instagram feed posts, Instagram carousels and Facebook feed.
Instagram re-encodes every image you upload — no tool can prevent that, and anyone promising otherwise is promising something they do not control. What you can control is what it re-encodes from.
1080px wide is the floor: below it, the upload gets scaled up before it gets compressed, and the text edges go soft. 1440px gives the compressor more to work with, which is why it is the default here. Either way the export is rendered from your original photo at full size, never by enlarging a preview.
Instagram locks the aspect ratio across a carousel: whatever the first slide is, the rest are cropped to match. Picking the shape once at the start — rather than discovering it on slide seven — is the whole reason it is a property of the carousel here and not of a slide. Up to 20 slides, all 4:5.