To change the size of a variable inside a loop you can just use a statement such as this:
length = length + 25
The program finds the old value for length, adds 25 to it, then saves the new value where it had the old value.
To make the square spiral start by drawing a short line turn and draw a longer line, turn and draw a longer line, turn and draw a longer line…
On your own #15:
Square Spiral
draw square spiral using repeat loop and a variable that changes