How can you make your print screens bigger?

That's a self-contradiction. A screenshot is a picture of your desktop area. In order for the screenshot to be larger than your monitor, the desktop would have to be larger than the monitor. The only way this is possible is to:

A. If running a CRT monitor, jack up the resolution higher than your monitor's supported max. The image would still technically not be larger than your monitor screen, but it would increase the total size of your screenshot. This can easily damage older monitors, so do it at your own risk.

B. Some LCD monitors support a "real estate scrolling" method by increasing the maximum resolution, similar to the above method. This displays only a portion of the desktop at a time.

C. Use multiple monitors.