Size was a measurement or amount of an individual or thing.
Anatomical size
The Talosian brain was three times the size of a Vulcan or Human brain. (TOS: "The Cage")
Clothing sizes
In 2375, Jean-Luc Picard, struggling with a tight Starfleet uniform collar, commented "My collar size is exactly as it was at the Academy." (Star Trek: Insurrection)
Kathryn Janeway wore clothing size four, which she revealed prior to taking on the role of Queen Arachnia (whose costume she would need to wear). (VOY: "Bride of Chaotica!")
Michael Sullivan claimed his boots were half a size too large, therefore his toes never crossed the line (a beer stain) while playing rings. (VOY: "Fair Haven")