Framed

Framed is a macOS menu bar app for quickly resizing visible windows to common aspect ratios.

Features

Lives entirely in the menu bar.
Lets you choose from 16:9, 9:16, 4:3, 3:4, and 1:1.
Keeps each window's current width by default, or sizes windows to a share of the screen width: 50%, 61.8% (golden ratio), or 95%.
Groups visible windows by app and applies the selected ratios to every window in the chosen group.
Uses macOS Accessibility APIs, so it needs Accessibility permission in System Settings.