Two squares is sometimes similar.
A square can be any size, as long as all four of it's sides are equal.
A square would be a rectangle if two of the sides were different than the rest.
A square would be a trapezoid if only one of it's sides were different.
For a square to be always similar, the ratio of it's sides must be equal to the ratio of the second square.
For example:
All of a square's sides are 5 across.
In order for another square to be ALWAYS similar, it's sides must be a multiple of (The length of the original square's sides) 5 (10,15,20,25,30...)
This is called a ratio. (5/10 = 1/2, 5/15 = 1/3, 5/20 = 1/4 etc...)