With InDesign CS5 you get a lovely new tool, the Gap tool. Basically it allows you to manipulate the gaps between boxes, thereby changing the size of the boxes themselves.
For example, if you layout eight rectangles and, using InDesign’s gap tool, you can change the position of the gaps between each set of frames, whilst the frames resize.
By default, it will move the gap that you have. However you can also…
…use the gap tool between frames and the edge of the page