DX11 plugin not allowing high-res

I'm stuck at 1x scaling on the DX11 plugin. The option appears to be greyed out. It's by no means unplayable, but I'd be better off just playing the game on the Dreamcast proper if there is no way to upscale.

Rig:
Ryzen 5 1600x
16 GB ram (DDR 2400)
EVGA GTX 1050 Ti SSC
Win 10