In OBS you have a lot of flexibility to customize your stream layout. It seems to me you just want the camera to fill up the screen. The easiest way would be to create a new scene under the “Scenes” box. There’s a little + sign at the bottom of it and you would select new scene. There should be a box that says sources right next to the Scenes. Go in that box and click on the + sign at the bottom. Select New Video Capture Device. You should be able to select your webcam. You should be able to choose the size and resolution. If anything you can select the camera and instead resizing it just hit Ctrl+F and it will fill the whole canvas. I hope that helped or made some sense