All the sizes..
 
So the first step in creating the DD is determining the size. I want to use a or the most common sizes, because this would lend to not only printing of a final version being easier, but also as works in progress, they can be printed and included in standard binders, very likely a binder you or I already have. Research is showing that the most common sizes  are 3.75"(inches)x 6.75"/3.7x6.7(Inches), 5.8"x8.3", and 8.3x11.7 , in descending order.  There's a slight advantage of using the slightly larger as to use less paper or hold more information. However, Filofax(tm), Kikki k(tm), DayRunner(tm), DayTimer(tm) . Franklink Covey(tm) uses sizes that are just so different, and just different enough  to make refill compatibility a challenge.  As a point of reference, the new age datebook my cousin "Lou Ellen" uses measures 5x8. 

Feedback is certainly welcome on preferred sizes, meanwhile I'm going to be experimenting on them.