The options (from top to bottom):
Given in
This selection determines the interpretation of the contents of the
size fields. You can choose between giving absolute sizes in pixels, in
percent or as factor, relative to the original size.
Size
Put the figures according to the "Given in" option, here.
Keep ratio
Determines, whether a change of the ratio (distortion) shall be
avoided.
Keep details
When scaling down images, information is lost. This can lead to
unwanted and annoying effects. Activate this option to include a
maximum amount of pixels into the calculation of the result image.
Perform
Starts calculation. The image/project is fixed before and all
undosteps are deleted.