Using this function you can change size of image and using some filters you can preserve image quality.
First step of resizing is to select type of resize - crop or resize :). Then you have to select new size of image - you can select one of four presets or input your own. If you have checked 'Preserve aspect ratio', image will have still the same Width:Height ratio - this applies on presets too! Last step is to select filter (just for resize). Filters are placed in the only dropdown box of dialog and they are sorted by quality (and speed). 'Windows' filter is fastest filter, but it's quality isn't very good. The most quality filters are Lanczos 3 and Hermite. Each filter is good for specific type of image - best way to find out your favourite is to test all filters...