Resize every image while keeping its proportions.
Width and height are pixel dimensions, not a file-size target. Without a fixed crop ratio, Resize fits the whole source inside the entered limits and keeps its proportions. Setting only one limit lets the other dimension follow the source ratio.
A fixed-ratio crop discards edge pixels to reach the selected shape; the preview lets you move the image before export. Fit keeps the whole picture and may not fill both target dimensions. Stretching a photo by forcing unrelated width and height would distort it, so check the preview and output numbers.
Inspect faces, text and thin lines after a large upscale: adding pixels cannot create captured detail. These edits are made in the browser, and the image files are not uploaded to a Pixlena server.