The canvas, you can start by resizing the canvas. When you see this symbol on the right or bottom part of the canvas, you can drag to zoom in or out. Next, you can resize the preview window. You can do this in the View menu, zoom in, zoom out, or using the keyboard shortcut command, equal or Command minus. You can also click on View, zoom to fit or zoom to 100% to get to the preview at the windows resolution, you should also adjust the size of the canvas if you have a snap to front window selected into the snap to the window in the video. You can also change the background color like this and reset everything to normal.

You can see in the Properties tab that the preview window moves. resizing is done by clicking on the little circles in the corners of the preview window using shift withholding The aspect ratio you're dragging. You can also rotate the item in the canvas by moving this circle