What effect, if any, does prop size have on timing?

   Timing of what?

      Brett

Well, good thing I clarified that. 
Anyhoo, the question arises due to a situation I encountered while bench running an O&R .60 S/P on glow, recently resurrected from parts engines.
It would start easily enough but would run ONLY backwards.
Running straight meth/castor and with no compression adjustment possible, I vaguely remembered reading something about prop size affecting timing so I wanted to clarify the relationship, as even a 14" prop made no difference.
However, in the event it turns out to be a non-issue, as I found my field box battery to have low voltage.
After overnight charging, the engine fires up very easily and runs normally.
It certainly seems counter-intuitive, so any thoughts on low voltage causing a pseudo-advanced condition?