If the floor of the cupboard is 10 inches square, this means that each of the four sides of the space on the floor is 10 inches in length, a square by definition having four equal sides. The description is of a two dimensional space. It says nothing about the height of the walls of the cupboard. Presumably they are tall enough for a child to stand (maybe 4-5 feet) but the limited space on the floor means that the cupboard is too small in which a child could move freely. This makes the cupboard a torture chamber. By the way, do not confuse "10 inch square" (a square with four sides of 10 inches each) with the deceptively similar expression "10 square inches." The latter refers to the space INSIDE of a geometric form. For example, a rectangle of 2 inches long by 5 inches wide has 10 square inches of space inside of it (2 x 5 = 10). A 10 inch square would have 100 square inches inside (10 x 10= 100). For instance, this description is useful when talking about how many square feet of floor tiles, carpet or wood one needs on a floor; one multiplies the length and width of the room to obtain the total "square footage." Again, this description is two dimensional and says nothing about height or volume. How much a box or other container holds is described in "cubic" inches, feet, yards, meters; this describes three dimensions. By the way, one says "ten inch square cake," (inch or other measurement in the singular, regardless of the number).

The first option is right - 3 1/3 in by 3 1/3 in or 5 by 2, etc.