As a beginner, I may be getting in a little deep. I am planning to build a shed either 12 x 24 feet or a little bigger (up to 16 x 30). Cost will determine which. I have a basic shed design out of a book, but would like to have a gambrel roof. I need some advice and resources. I was thinking that using continous purlin plates to join the two slopes might be an idea, but don't know about sizing the plates and attaching the rafters. Any advice will be appreciated.

You might check out Stewart Eliot's Timber Frame Planning Book. He has some gambrel plans which would give you an idea of frame timbers. Not certain that you can still get the book.

at this size your main wall plates will be around 8x8 and the purlins 6x8...7x7 or so.
Step lap the main plate and bird's mouth the purlin?
-Mark.