How can I change printing size in Prism?

There is no way to change size when printing, but you can change the size of a graph in the Graph section. Choose "Change... Graph Size and Shape" (ver 3) or "Change... Size & Frame" (ver 4). This won't affect symbol size and font size, which you can adjust separately.

What is better, make a page layout of just that one graph. Click on the graph (in the layout) and drag the corner to make it larger or smaller. Everything will change proportionately.