Upload limits
Guests and free accounts share a generous per-file cap that comfortably covers phone photos, RAW files and multi-page documents. Premium raises it for large TIFF scans, PSB composites and long PDFs. The exact figure is shown on the upload panel of every tool.
WebP's dimension ceiling
The WebP bitstream stores width and height as 14-bit values, so neither edge can exceed 16383 pixels. This is a property of the format, not of this site. Stitched panoramas, 1200 DPI flatbed scans and large-format TIFF are the usual sources that hit it.
What happens if you exceed it
Rather than failing with an opaque encoder error, oversized images are scaled down proportionally to fit within 16383 pixels on the long edge. For everything short of genuine gigapixel work, this ceiling never comes up.
Reducing size before upload
If you are near the upload cap, resizing to the dimensions you actually need is far more effective than lowering quality — halving both edges quarters the pixel count.