A print-on-demand design file fails an upload for one of two reasons: it is too large for the platform's byte cap, or it is too large in pixels, which is a different number entirely. Printful caps files in its library at 200 MB and 20,000 x 20,000 px. Printify caps JPEG and PNG at 100 MB, and SVG at 20 MB. Redbubble refuses anything above 300 MB or 13,500 x 13,500 px. Those three caps are 100 MB apart and none of them is about quality — they are server-load numbers. What actually makes a print look like a print is DPI at the real print size: Printful wants 150 DPI for most apparel and 300 DPI for paper, stickers and phone cases, and Printify recommends 300 DPI for PNG and JPEG but drops to 120-150 DPI for large-format items like leggings and blankets. So the safe workflow is: size the canvas to the product's print area, do not upscale a small original, then compress the file without touching the pixel dimensions. Skwish compresses to a KB or MB target, resizes to exact dimensions, and reports the before and after size per file, all in the browser. One more thing worth knowing: Printful warns that Photoshop's Save for Web often drops a file to 72 DPI and prints grainy — that is an export mistake, not a compression one.