You can change the size of the whole project or only one video of it.
Choose a necessary size in the upper menu of the Editor clicking on the “Canvas size” button in order to change frame width and height of your project .
You can also change proportions of only one video of your project in the following way:
Move the red frame of your video or image to set necessary ratios e.g. square:
Use the “Actions” menu and choose the “Fit” tool and then “Cover” one.
Your video will have a chosen frame.