Size means two different things and people are rarely asking about the one you assume. This is about megabytes, not pixels.
Choose Keep original size if pixel dimensions must remain unchanged. Compare lower-quality JPG or WebP outputs, and check any separate pixel requirements. A smaller byte count alone does not guarantee acceptance.
The starting maximum width is 1920 pixels. Choose Keep original size when dimensions must remain unchanged.
JPG and WebP use the quality control. PNG ignores it; compare actual sizes because results can be larger.
Set Maximum width to Keep original size and lower the quality instead. Between 65 and 80 percent is where most photographs still look the same to the eye, and the file can be a fraction of its original size. WebP gives a smaller file than JPG at the same look.
A PNG will not shrink much, because quality has no effect on it. If it is a photograph, convert it to JPG or WebP first. Each file is shown with its before and after size, so you can step the quality up or down until you are happy.
Choose Keep original size instead of the default maximum width. JPG or WebP quality changes can reduce bytes but also affect visual detail.
JPG, PNG and WebP always open. A phone may run out of memory with a big batch of large photos, so add fewer pictures at a time or use a computer.
Between 70 and 80 percent is safe for most photographs. Go lower only for pictures that will be seen small.