I made a graph with two or three axis segments, but Prism keeps changing the size of the graph. It won't stay the size I want.

This happens with Prism 4 when you have data points that fall between axis segments so don't appear on the graph at all. Instead of just ignoring these points as it should, Prism gets confused by them. The workarounds are either to exclude these points from your data table, or change the limits of your axis segments so all data are plotted. Prism 5 does not have this problem.

Note that Prism 4 also gets confused by curves that have values within the gap. Go to the Curve page of the nonlinear regression results, and exclude the values that fall in the gap. In the phrase of the London Underground: Mind The Gap!