For figuring out what size secondary you want to use (there is no exact size for any particular basic design in this field) go find a copy of NEWT25.EXE (for the PC) on the web. It will go through all of the calculations for you and show you what is important - you just figure out which is important to you.
As far as Orion Optics are concerned, they are good middle of the road optics sets and very rarely do you get some bad optics from them. For a first scope, they will do very nicely and the only way to get better optics for the price is to do them yourself which, while it`s not all that hard to do, could be daunting to someone who hasn`t done that sort of stuff yet.
Sorry for the late answer but I just got back from RTMC which was wonderful as always.