"Size" is ambiguous, so I'm giving you two answers.
When you hear that a TV is 40 inches, the length from one corner to the opposite corner diagonally is 40 inches. To solve, we use formula for hypotenuse, a^2 + b^2 = c^2.
20^2 + 25^2 = 32.02^2.
If that is what they mean by size, your answer is 32.02. However, size could refer to the area of the screen, which is calculated by length * width.
20 * 25 = 500
The answer in this case is 500.