1Open Image Size

Click this button to open the dialog window for image size.
This function can be found under Image -> Image Size

2Checkbox: Duplicate Image

Select this checkbox to duplicate the image beforehand.
This function can be found under Image -> Duplicate...

3Checkbox: Flatten to Background Layer

Select this checkbox to flatten all layers and groups to the background.
This function can be found under Layer -> Flatten to Background

4Checkbox: Copy to Clipboard

Select this checkbox to copy the modified image to the clipboard.

5Enter Value

Please enter the desired value here.

6Pixels or Percent

Select whether you want to specify the value in pixels or percent.
If you use pixels, the larger side will be scaled.
Holding down the Shift key while using #7 will scale the smaller side.

7Scale Image

Click this button and the image will be scaled with the desired values.

Shift Key

If you hold down the Shift key while using pixels, the smaller side will be scaled.

TIPRight Click

A right-click on button #7 shows you this available function, so you don't have to remember the keyboard shortcuts.