Some portals cap uploads at 150 KB, usually for scanned documents and photos. At this size a full-resolution phone photo comes down to around 1200 px and still prints fine at 10×15 cm. Scans of documents keep every letter readable if you crop away the borders first.
At 150 KB you can expect about 1100 to 1300 px on the long side. The exact number depends on how much fine detail the image has: skies and plain walls compress well, foliage and fabric don't.
Tip: Scanned documents compress much better as JPEG at quality 70 than as PNG. Choose JPG in More options.