Clinton,
There's no reason, either structural or tonal, for the top of the harp arm to be a continuation of any "doming" of the main guitar top - you want it's top surface be on a similar plane to the fretted neck, and for aesthetic reasons you can continue the longitudinal arch of the back into the harp guitar arm's bottom piece if you want to.
The scale length of the sub-basses is given by the string gauges, tunings, feel and sound that you want. For harp guitars with 5 or 6 sub basses the lowest string can be G or lower.
Unless Shane has found some very strange shaped trees, to get a bookmatched top both pieces are the same size. Don't worry though - you can make some great instruments out of the piece you don't need for a harp guitar - I made a lovely ladder braced concert sized guitar from mine.
_________________
Dave White
De Faoite Stringed Instruments". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010