The windows version of Tux paint has a configuration program to help you set among many other things resolution of the program. For some reason the Linux version doesn’t have this.
I have found a solution using the man page you seem to have to options manually when running, (you can then edit the appropriate menu entry and each time you launch the program it can be told to run with those settings)
If you are using xubuntu you can edit the menu system with menu editor, found within settings manager.
Once loaded navigate to where tux paint entry is and you see the following, I have modified the entry to tux paint already. If you click on image below you can see where i have added –800×600 after tuxpaint in the command box. You can set a variety of options running man tuxpaint gives you a full list of options.
How you edit the settings really does depend on your distribution however.
Any questions please e-mail me or tweet @zleap14