The Big Card has the ability to make objects and creatures bigger in size. She is under The Light's jurisdiction and by extension, Kero's. The Big is portrayed as a generally-harmless card who likes to make objects around her bigger.
Its physical form is that of a tall slender blue woman, dressed like a medieval princess with a divided hennin. Big is almost an exact copy of its sister card, The Little, except for color and height.
The Big card has the power to grow any person or object in size with its wand.
Big first appeared in Episode 31 where it was captured with The Create, who was sealed later in the same episode. Sakura first invokes the power of The Big to maker herself larger in order to subdue a colossal dragon Naoko Yanagisawa inadvertently brought to life with The Create while it's powers were unsealed.
It appears again in Episode 55, when it was transformed, where Sakura used it to restore herself to her normal size after being shrunk.