Since the block is made from jelly rolls, and jelly rolls are 2.5 inches wide, the first square is a 2.5-inch...well, square. For the second color, you would cut one piece at the same size (a 2.5-inch square) and one more piece 2 inches longer (4.5 inches). Then, for each subsequent color, you cut one the same as the longest side you just sewed before, and another 2 inches longer than that. Ad infinitum, if you're into that.
Since I did seven colors, I ended up with a big-ass block. How big? Pretty damn big. Hmmm...let's se if we can find something to put next to it. To, you know, give it scale. Oh, hey, thanks kids. That's perfect.