You may need to hit the 'edit scene' button and resize your scene elements.
Also be aware that Twitch *expects* a 16:9 stream, and will letterbox it if you hand over anything else.
It's an option to set 1920x1080 in the 'custom' section of OBS, and scale/move your elements around... won't eliminate the letterboxing, but it will give you control over it and allow you to push the game to one side (making room for an onscreen chatbox and/or webcam) or overlay a frame graphic to fill in the empty space with content.
You CAN use 'shift' to enable non-ratio resizing, but this isn't recommended... it tends to look pretty bad, in the end.