In Management Reporter there is no way to change the default page setup except by editing each Report Definition.
This is an issue in the UK because the default page size here is A4 rather than Letter (as is common in the US). This means that every single new report created needs to be edited to have an A4 page size.
I think it would be incredibly useful if there was a global page setup which could be defined and would be the default for every single new report created in Management Reporter. Not only would it allow for a default page size, but also allow for a global default margin size or to choose between portrait and landscape as the default orientation.
If you think this suggestion is a good one, then please vote here and tell Microsoft how many people would benefit.