This is a security.
The allowed dimensions are stored on the top of the image class file.
It is not the best place to set up the allowed dimensions, but for the moment I didn't rewrited this class yet, it is old code but working nice and stable.
Why such security ?
1. Because a thumb file is created for each size (hidden file), so if somebody want to ennoy you, he just need a script which call an infinity of combinaisons, and fill you server.
2. Because if the size is too big, it will consume a lot of CPU.
This one is easy to fix.
Open :
classes / image.php
Add some sizes in the constant in top of the class.