Hi Stacey,
I am also awaiting a 16" SDM build due sometime early next year. I have rocked back and forth between 14-16" apertures. Initially I had settled on a 14.5", thinking it was small and light. Only recently it was changed to a 16". They are both LARGE size dobs and you will need to wheel both scopes around. Peter Read has told me several times that it's easier to wheel a 20"+ scope than to carry a 12" dob!
Hence bulk/weight shouldn't be a factor between these sizes. If you want something smaller and fit into a sedan, then your limit is most likely a 12.5" (just). I think more importantly to consider is costs and focal length. If the costs permit, go the 16" 4.5
Or even better, an 18" f4
