Walk-in closets vary in size and shape. But the most desirable size for a walk-in closet in a master bedroom is at least 100 square feet.
This sizing assumes that there are two full-time residents using the closet. If more or fewer people will be storing clothing in the closet, then the square footage should be adjusted to match.
With that said, a walk-in closet can be constructed within as little as 25 square feet. And there’s nothing stopping you from adding the amenities of a walk-in closet (standalone lighting, diverse storage options, etc.) to a standard-sized closet!