The “100 step rule" is perhaps one known dimension.
This relates to the known time for neuron-neuron interaction and the measured human response time in various experiments.
http://onlinelibrary.wiley.com/doi/10.1207/s15516709cog0603_1/abstract
On the other end of the timescales I hypothesize this:
I propose that a single thalamocortical resonance cycle in the cortex is the smallest quanta of human perception.
I propose that a single thalamocortical resonance cycle in the episodic portions of the cortex (those connected to the hippocampus) is the smallest quanta of human experience.