The logic of the align palette

How does the align palette decide which objects to change, and which ones to leave the same?

For instance, say I have five rectangles on stage. One is 20x20, the other four are 50x50. I want to use the align palette to match the size of the four 50x50 rectangles down to the 20x20 rectangle. My desired result is five 20x20 rectangles.

How do I make the 20x20 rectangle my ‘anchor’? In other words, my 20x20 stays the same and my four 50x50 rectangles shrink down to 20x20.