Maybe it is a newbie question, but I don't know how and where to set page size?
What is the difference between Generated Design and AutoDesignSpecs?
Why do I find the same object (label, text box etc) under either sections?
To set your page size right-click the design node of your report, then choose "printer setup" to select the printer and "page setup" to choose paper and orientation.
By googling around:
With the AutoDesignSpecs you give AX metadata of what you want to put in the report, and AX renders this a runtime to get the layout. This is a very flexible way to declare a report and should be used for most regular reports.
With the GeneratedDesign you create the layout and control where to put everything. This is something you'd need for documents like an invoice or a packing slip.
You find the same object names in both because the generated design was initially created from the auto design yielding the same node names.
Also see Best Practices.