Go to https://pixlr.com/x/
Click [OPEN IMAGE] and select the file from your computer,
Click the “Arrange” icon
Click [Transform] then enter the new width or height in pixels. Once you have the correct size click [Save].
The download dialogue box will open when [Save] is pressed.
If saving for the web, reduce the quality using the slider as much as you can without the image distorting (The medium preset works well) and then hit download.
The image will download to your default download location.