While Squish showed a quick solution, it is not suitable for all needs.
Imagine you would want to set the sprite's dimension to 50% and a few seconds later to 75%. Let's say the original dimensions were 100x100. Then we would want the sprite to first shrink to 50x50 and later grow to 75x75.
1) width = width * 0.5 (the sprite is now 50x50)
2) width = width * 0.75 (the sprite is now 37.5x37.5)
You could keep track of when to grow and when to shrink, and calculating that 75% is 1.5 * 50% and therefore replace 2) with width = width * 1.5, but it gets more and more complex as you change the sizes.
An easier way is to use the expressions Get original width/Get original height. They always return the original initial values regardless of any changes to the size.
1) width = OriginalWidth * 0.5 (sprite is now 50x50)
2) width = OriginalWidth * 0.75 (sprite is now 75x75)