How to set a window to an exact size and position on a Mac

Does macOS have preset window sizes?

Only relative ones. Window, Move & Resize offers halves, quarters, fill and centre, which depend on the screen. For an exact number of points you need a script.

How do I set an exact size with a script?

This sets the front window of the front app to 1280 by 800 points, with its top left corner at 100, 60:

osascript -e 'tell application "System Events"' -e 'tell (first process whose frontmost is true)' -e 'set position of window 1 to {100, 60}' -e 'set size of window 1 to {1280, 800}' -e 'end tell' -e 'end tell'

The first time, macOS asks to let the app running it, such as Terminal, control your computer. Allow it in System Settings, Privacy & Security, Accessibility.

How do I run it with a key?

1. Make a new Shortcut with the Run AppleScript action and paste the script below in place of its sample text.
2. Give the Shortcut a keyboard shortcut. See run a Shortcut with a key.
3. Switch to any window and press the key.

tell application "System Events"
  tell (first process whose frontmost is true)
    set position of window 1 to {100, 60}
    set size of window 1 to {1280, 800}
  end tell
end tell

Why does a window not reach the size I set?

Apps can set a minimum or maximum size, and some fixed size windows cannot be resized at all. A window manager may also move it straight back to its place.

Finder only: Finder window size. Recording at that size: record one window. Resizing by keyboard: resize windows with the keyboard.