The only thing that matters is that the resolution is the same on all monitors, which you can easily make sure of in your application. If the monitor is in 800x600 resolution, no matter what actual size it is, it means it is 800 pixels long and 600 pixels wide and the actual numbers you are using to display the pictures represent pixels. If you always use the same resolution, pictures on bigger monitors will appear bigger since each pixel would be a little bit bigger on a bigger monitor. Take a look at http://java.sun.com/docs/books/tutorial/extra/fullscreen/index.html
for an easy way of setting the resolution. Basically, if you make sure to switch to a certain resolution at the start of your application, if it looks good on your monitor, it should
look good on all monitors.