I don't have mine activated, so I can't check, but you can check the Preferences in Facetime to see if there is an option. If not, then it does not support full screen - you can most likely drag on a corner to enlarge it to about the size of your screen.
And, yes, typing in all caps is not necessary; it's much easier to read and we can hear you much better without it.
(1) Launch FaceTime
(2) Connect with one of your contacts.
(You must be connected to activate full screen view.)
(3) As soon as you have established two-way video, the controls appear at the bottom of the picture like this:
If the controls go away, hover your mouse in the bottom of the picture. The controls will reappear.
(4) Click the diagonal double arrows at the right side of the control box to enter full screen view:
(5) Clicking the diagonal double arrows again or pressing <esc> (Escape) cancels full screen view.
(6) If the above doesn't work, restart Mac and try again. Unless your software is damaged, the above works.