Johno_71: As homework or as a project?
If a project, the output voltage of the transformer at the maximum current load has to be at least one forward biased junction drop above the desired output voltage for a series regulator. The maximum can be anything higher. If you are looking at large voltage differential, a phase control pre-regulator is in order followed by a standard series regulator. I find analog regulators easier to design and digital more efficient but, it requires more filtering and will usually need a slow start circuit to get the caps up to voltage.