how to get facetime bigger on the mac air

If you place your cursor onto the FaceTime window you should also see a set of controls appear at the bottom or left-bottom of the screen. There is a full-screen button. Click on it to expand to full screen. You also can use the corner, side, and bottom gadgets to expand the screen. Just place the cursor on the edges of the screen. It will change to an arrow gadget you can use to adjust window size.

You will see them once you connect to someone. They do not appear on the open window.

Use FaceTime on your Mac

Welcome to FaceTime on Mac - Apple Support