Etsy recommends at least 2000 pixels on width and height, and warns that files over 1 MB may not finish uploading. Those two pull against each other, so large phone photos often need compressing. This tool shows where your photo stands.
Etsy also says a first photo under 635 pixels can show up lower in searches. Your photo is read in your browser and never uploaded.
Choosing a first photo for Etsy
Etsy advises a landscape or square first photo, so the focal point stays in view when thumbnails are cropped. This checker does not judge the crop, so keep your product near the centre with some room around it. Avoid transparent PNGs too: Etsy shows those areas as black.