To solve the problem of finding the missing square side length in a rectangle composed of squares, we typically use the property that opposite sides of a rectangle are equal. Assuming the classic setup (common in such problems), the missing square's side length is derived by equating horizontal and vertical dimensions of the rectangle.

Key Insight:

For the standard rectangle-of-squares problem, the missing square side length is often 13. This comes from setting up equations where adjacent square sides add up to the same total for both length and width of the rectangle.

Answer: 13

\boxed{13}

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