On camera, size is the actor transformed into the character. It looks like they belong in that space, commanding attention.
If you look size up in the dictionary, the fourth or fifth definition is character, quality, or status of a person or thing, especially with reference to importance, relative merit, or correspondence to needs.
When you are acting, this is what your character needs to show to have size. I need to see precisely your character’s quality as a being, where they belong in the hierarchy of their world, and how this drives them to obtain what they need and why it is essential to them as a character.
All this boils down to, “Who is the character?” Not an existential, analytical, metaphysical who, but an autobiographical examination of the character you’re about to play. Start with the simple idea of what is the character trying to accomplish, be it a man trying to find his father, or a woman traveling through space in search of life.
I can still hear Stella yelling “Size! Size! Size!” She stopped me while I was performing a monologue from Romeo and Juliet. “Stop!” She yelled, “you’re butchering it! Stop!” I had barely started. All I had spoken was, “But soft, what light through yonder window breaks.” “Who are you talking to?” Stella demanded. “What do you see? Where are you in the scene? Where are the stakes? I need to see life and death!”
Once you have this necessary foundation of the character, you can then begin building upon it with details that support who you are.
This is how an actor obtains size through the technique.