There is an option to unlock the slider at a small hit in visual quality. Initially the slider is locked at certain increments (150%, 200%, 250%), this ensures optimal pixel locations when downsampling and thus the best quality. Quality SSAA downsampling occurs at values above 100%.
The configuration itself is really simple, you just have to choose a scaling value by adjusting the slider (from 25% to 300% 500% your native resolution) and clicking Apply. While in-game press F10 or Left Ctrl+R to open the mod configuration window.
