How do I change image size in Serif Photoplus 6.0?

[BeckyTheStrange]

I downloaded Serif Photoplus 6.0 because an awesome site (www.xentrik.net) recommended it. However, I can't seem to change the image size. Every time I click on a box to enter information, a popup comes up telling me that I need to input a number. Then when I finally input information (in, let's say, the "image size" box), the information in the "print size" box turns negative and another popup comes up! What can I do?