In the main Creator window, go to the menu and select Tools | Options. This calls up the General Options dialog. Go to the Machine tab. That's where you define the dimensions for your machine.
Also, after you've imported and arranged your model and added a process, you can override the bed dimensions. Double-click the Process name to open the FFF Settings window. If it is not already selected, choose the Advanced view (button on bottom left), then go to the G-code tab. There is a selection there where you can override your machine dimensions for the current process. The checkbox is normally unchecked, but if you check it, you override the limits set in the General Options with the dimensions you enter here.
I hope this is what you are looking for.