Write down the length of one of the square's sides. If you must, measure the side with a ruler to find out its length.
Select a scale factor. For instance, if you want to create a square that is five times as large as the original square, then your scale factor would be 5.
Multiply the scale factor times the length of a leg to find the new length of each leg. For example, if the original square has a leg with a length of 2 inches, you would multiply 2 times 5 to get 10 inches as the length of all four legs for the new square.
Draw the new square using a ruler if required. The angles will still be the same, since square by definition have four 90-degree angles. The legs will each be the length that you calculated in Step 3.