Area Select Tool

Area Select Tool is a rectangular area selection tool.
hotkey: M

Modifier Keys
Ctrl - hold to Add to an existing selection.
Alt - hold to Subtract from an existing selection.

Tool Options
Make Selection of specific Width and Height dimensions:
Use dedicated W and H fields for width and height, and press
[ Make Selection ] button.
This will create a rectangular selection of secified dimensions (in pixels)
Aspect ratio lock helps creating selections of width and height dimensions relative to one another. Using W and H fields, provide ratio dimensions and activate by checking the Aspect Lock checkbox.
e.g. 1 and 1 would be a square, or another example 16 and 9 is a standard ratio for many displays.
Additionally moving any active selection with a cursor, while Area Select Tool is in use - will keep selection bound to canvas boundary.

Selection continued...
More information on Selection feature here