Pixel size and file size are different. A 4000 pixel photo can be several megabytes. The same picture at 1280 pixels wide is usually enough for a full-width website image, and 800 pixels is enough for a card.
Aspect ratio stays locked by default. Enter a width and the height follows the original shape. If you enter both, the picture fits inside that box instead of stretching. Turn the lock off only when you mean to change the shape.
Upscaling is off by default. Making a small photo larger does not add detail, so the tool will not enlarge it unless you allow that. Images larger than 4096 pixels on either side are refused, so the page does not try to hold an unbounded bitmap in memory.
1. Open Resize and choose the image.
2. Enter the width you need, or both width and height.
3. Leave “Keep aspect ratio” on unless the destination requires a different shape.
4. Check the output dimensions, then download.