An upload form asking for a smaller image can mean fewer pixels, fewer bytes or both. Resizing changes the pixel dimensions. It can reduce file size too, but that second result depends on the format and encoding choices.
Start with Image Resizer when you know the required width. The ratio lock keeps the original shape rather than forcing the picture into an unrelated rectangle.
Let the ratio calculate the height
A 1200 by 800 image resized to a width of 600 becomes 600 by 400 with the ratio locked. The height field is ignored in that mode and disabled in the browser interface.
Unlock the ratio only when you deliberately want the exact entered width and height. Forcing that same source into 600 by 600 stretches its proportions. A face can look wider or taller even though the dimensions satisfy a form.
If you need a square without distortion, choose a crop or a padded canvas instead. Image Cover Crop fills a target by cropping, while Fit Image Inside Box keeps the original image within a box.
Inspect the result at a useful size
Enlarging a small image cannot recover detail that was not in the source. A 300 pixel picture expanded to 1200 pixels is larger, but not equivalent to an original 1200 pixel photograph.
Choose PNG when transparency must remain. JPEG requires a solid background, while WebP depends on host support. PNG quality is not adjusted by the lossy quality field.
You can apply the same width settings to a batch, with separate generated files and a combined ZIP download. Different source ratios will produce different heights under the lock.
Check the final pixel dimensions and download bytes before uploading the result elsewhere. A form's dimension rule and its size allowance are separate checks, and passing one does not guarantee passing the other.