Fill, fit or stretch
Resizing to an exact size only has an obvious answer when the image already has the right proportions. When it does not — and it usually does not — something has to give, and the three options give away different things.
Fill and crop scales the image until it covers the frame, then trims the overflow from the edges. Proportions are preserved and nothing is distorted, but you lose whatever fell outside. This is what social platforms do to your uploads, and it is the right default for them.
Fit inside scales until the whole image is visible and fills the remainder with a colour you choose. Nothing is lost and nothing is distorted, at the cost of bars along two edges. Use it when the entire frame matters — a product photo, a diagram, a screenshot.
Stretch forces the image to the exact dimensions regardless of its shape. It is almost always wrong, and it is included because occasionally it is exactly what someone needs.
Enlarging does not add detail
Scaling an image up cannot invent information that was never captured. A 400-pixel logo enlarged to 1200 pixels is the same 400 pixels of detail spread across nine times the area, which reads as softness. Good resampling — this tool uses Lanczos — makes the softness look natural rather than blocky, but it is still softness.
The tool says so when it happens rather than quietly handing back a blurry file. If you have any larger original, start from that instead; if you do not, an AI upscaler is the only thing that will genuinely add plausible detail, and it invents rather than recovers it.
Resize before you compress
When a file needs to be much smaller, resizing does far more than compression and does it without artefacts. Halving both dimensions removes three quarters of the pixels; achieving the same reduction by pushing quality down would visibly damage the image. Resize to the largest size you actually need, then compress to hit a byte budget.
If you want to choose what stays in frame rather than letting the fill mode centre it, crop first: crop to any ratio, or use the circle crop for a round avatar with transparent corners.