Welcome to FXhome, Silencer!
Download the demo. Import your footage that was shot on the chroma-key color (red, blue, or green.) Go to the object mode. Select the Color Difference mode under the "key" pallette. Select your color, and the white settings, dedpending on how well it keyed. Then, go to the "Animate" pallette and a red border should appear around the keyed footage. Then, click, hold, and drag the boxes that appear on the border to resize it. Tip: If you have a green or blue border or aura-looking type thing around your actor, use the spill suppression.