Open one product image and see its exact dimensions and file size compared against the size rules for Etsy, Amazon, eBay and Shopify.
Every marketplace sets its own image sizes, and an image that is fine on one can be too small for zoom on another. This checker reads the selected image locally, pixel dimensions and file size, never uploaded, and compares it in one table: Etsy recommends 2000px or more on the longest edge for zoom with a 570px listing minimum; Amazon requires 1000px or more on the longest edge for zoom, prefers 2000px or more, and caps images at 10000px; eBay requires 500px minimum and recommends 1600 x 1600; Shopify recommends 2048 x 2048 square with a maximum of 4472 x 4472 or 20 megapixels. Each platform gets a pass, ok-with-warning or does-not-pass status with the reason. The figures are the widely published requirements at the time of writing, each marketplace makes the final decision and can change its rules, so confirm against its current seller help pages before a large upload.
Why does the same image pass on one marketplace and warn on another? Because the requirements differ. A 1200px image clears eBay's 500px minimum and Amazon's 1000px zoom threshold, but falls short of Etsy's 2000px zoom recommendation and Shopify's 2048 x 2048 recommended square. The table shows each platform separately for that reason.
Can a small image simply be resized larger to pass? It would pass a pixel count, but upscaling adds no detail, so zoom would look soft and some marketplaces assess image quality as well as size. Start from the largest original available and resize down to the needed dimensions instead.