Put simply, you don't. Not without severely skewing your image, at least. Your aspect ratio does not match the unmaximized twitch embedded player. OBS' preview can resize. Their embedded player can't. If you want to get rid of the padding on the player in its unmaximized state, you should use a 16:9 monitor. Or setting your monitor to a 16:9 resolution and padding your own monitor.
And when I say "unmaximized", I mean when you're watching it from the browser window without hitting the 'maximize' button on the player. If you hit the 'maximize' button on the twitch embedded player, it will do its best to fit to your screen. If the aspect ratios max for the viewer and the streamer, there will be no padding. The majority of viewers use 16:9 resolution, so the majotiry of them will experience that padding no matter what.
