The Window ‣ Resize Document Prompter menu sets the size of the prompter content when single-screen prompting and the Prompt in a window preference are used together. Its typical use is with streaming apps, when you need to stream content at a particular size.

This feature specifically sizes the prompter area in a window. The window size itself will be larger to account for its titlebar and toolbar. This removes guesswork and makes it easier to use with capture programs to stream an exact-sized prompter without any window chrome.

The size list can also be customized with the Edit Sizes… item:

Preset sizes window

After sizing a window, you may want to prevent the system from resizing it around the Dock and other screen elements when moved (this is automatic macOS window behavior). You can prevent window resizing by choosing Window ‣ Lock Window Resizing. A common case for this is if you need to resize a window larger than can be displayed (which is not recommended, but doable).

Finally, holding Option before selecting an item resizes all open windows. The same goes for locking window resizing.

See also: Saving the default window size