Select Image › Scale Image to change the overall size of an image.
Select the new size by entering it in Width or Height.
To change the proportions of the image when scaling (this distorts the image), click the chain icon to the right of the fields to break the link between them. When those fields are linked, all values are changed proportionately. Adjust the resolution with X resolution and Y resolution.
The Interpolation option controls the quality of the resulting image. The default Cubic interpolation method usually is a good standard to use.
When you are finished, click Scale.