Paint Shop Pro :: Canvas Size On X4

I've been using PSP for a long time now (fisrtly PSP7 and now PSP9) and I wanted to buy the new version PSPX4. I downloaded the free-trial version and I saw that it is impossible to generate files bigger than 10'000x10'000 pixels. In the canvas size menu, it's impossible to give a higher number. The problem is that a made bigger files with PSP9 and I'd like to keep on working on it without resizing them.

So my question is : Is it only a limitation of the trial version or is it simply impossible to work with bigger images than 10k x 10k ? I hope there is a simple solution because PSP9 encounters some problems with Win7...