How do I determine what resolution Windowed mode is using?

I’m trying to capture the game with OBS in order to stream it on twitch, and I’d like to set my capture resolution to the Windowed resolution so I don’t have to play the game fullscreen or do any resolution scaling to make it look good on Twitch.

The default window size is 1280x720. If you resize the window, the resolution scales pixel-perfectly so the actual resolution will change in that case.

Odd. When I use that resolution in OBS, I get about 2/3rds of the screen. Cuts off on the right just after the title of the game.

it might be relevant to note that I am using two monitors with nonstandard resolution. My primary monitor is an old TN panel at 2048x1152 (16:9). My secondary monitor is a newer Dell IPS at 1920x1200 (16:10). The game starts on the 1st monitor no matter where I position steam to launch it.

I have attached several screenshots showing the area being displayed by OBS in preview and my OBS settings