Pixels, percentage or a preset?
Use pixels when a website, form or app tells you the size it wants, for example 600 × 600 for a profile photo. Use a percentage when you simply want everything smaller, such as 50% to halve both sides. The presets cover the sizes people ask for most: square Instagram posts, Full HD screens, YouTube thumbnails, and 1200 or 800 pixels wide for websites and email.
With Keep aspect ratio on, the image is scaled to fit inside the width and height you give, so nothing gets squashed. If you need an exact shape, such as a square from a landscape photo, crop it first with the crop tool and then resize.
Why resizing makes files so much smaller
A modern phone photo is often 4000 pixels wide, far more than any screen needs. Halving the width and height cuts the number of pixels to a quarter, which usually shrinks the file by 70% or more before any compression. That makes resizing the quickest way to get a photo under an upload limit while it still looks sharp.
Making an image larger than the original is possible, but it cannot add detail that was never captured, so enlarged images look soft. For the best quality, start from the biggest version of the photo you have.
Private and fast on any device
Resizing runs in your browser using your device's own graphics, so there is no upload and no waiting in a queue. It works the same on a phone, tablet or computer, and location data stored inside photos is removed from the files you download.