There are 2 ways to resize the image: Resize Image (which will scale all the content accordingly), or Resize Canvas (which will only adjust the canvas size without scaling any content). Both of these options are in the Edit menu, and both open up similar dialogs.
The following fields are the same regardless of which method you use.Units
Select the units you would like to use for specifying the width and height.Width
The new width of the image in your selected units.Height
The new height of the image in your selected units.Keep Aspect Ratio
If this is selected, your changes to one dimension will change the other so that the overall width to height ratio of the image is preserved.Reset Button
If clicked, values set in Height and Width field will be reset to the current Image/Canvas dimensions.
The options specific to each method are listed below.
Pixels per Unit
The number of pixels used to represent one of your selected units (not applicable when units are pixels).
Since resizing the canvas does not scale the elements, you can choose the anchor point that determines where the elements should be positioned in the newly resized canvas. There are buttons corresponding to 9 page positions (indicated by arrows). For example, if you make both dimensions larger and select the middle anchor point (the default), then the canvas will expand equally on all sides of the existing content. If you choose top left, then the canvas will expand to below and to the right of the existing content (everything was "anchored" to the top left of the canvas).