If you play the game mostly in fullscreen mode, you can choose a lower resolution from
Options (Alt+O) >> Display >> Performance >> Full Screen UI Resolution
This won't restrict your view onto the 3D game world. Your extent of view will be the same as before, but the UI will be enlarged by the reduced resolution. When you switch to windowed mode the UI will use its native (high-res) scale.
Selected fullscreen resolution matches monitor resolution:
Same scene, selected fullscreen resolution of 1024 x 768: