When I create a layout, how do I keep each graph the same size as on the graph sheet?

When assigning a graph to a layout, click the button "Show Size & Position". The dialog enlarges to expose a new radio button--choose to "Keep it exactly the same size as it is in the graph section".

There is a catch--Prism will keep the graph at exactly the same size only if the placeholder on the layout is larger than the graph. Prism will then shrink the placeholder rather than stretch the graph. But if the placeholder is smaller than the graph, Prism prefers to shrink the graph rather than stretch the placeholder. So the solution is to make the placeholder very large before assigning the graph to it.

With Prism 3.0cx, this feature (keeping graph same size) doesn't work. You'll need to stretch the graph within the layout to get the size you want.