How minute does a lie have to be before it doesn't really count as one?

[Spock]

I think the answer lies (forgive me) with the state of mind of the supposed liar. Mens Rea (guilty mind), generally a term used exclusively in the criminal justice system for determining whether a crime has been committed (similar to what The Silver Stag was getting at) could be applied to discover whether a lie has been said.

If you knew that what you are saying is false and has measurable consequences then that might be taken as a lie. This can change depending on context. For example telling someone the wrong time innocently and causing them to miss their appointment or bus, even if only by a minute would not be viewed as a lie as you did not give thought to the consequences even a minute difference would make.

A strictly positivist approach to the question would tell us that any spoken inaccuracy is a lie with no consideration for the intent or state of mind of the liar.