The %7 refers to the 7'th output value, Out[7]. It is usually a bad idea to reference output by number since the numbers can change depending on the order in which cells are evaluated. That is probably why it is not working as expected.You can fix it by evaluating the cells in the order they are evaluated in the book. Or, use another list of numbers to specify the size e.g.
Row[Graphics[{RandomColor[], Disk[]}, ImageSize -> 5 #] & /@ RandomReal[{1, 15}, 10]]
