Rule of Thumb for Multi Family Unit Square Footage?

[Lucian Guadagnoli]

Good Afternoon BP forum!  

Is there a rule of thumb for how much square footage is 'typical' for a Xbr/Yba unit in a residential multi-family?  I realize this is highly subjective with a lot of variance, but there must be some average square footage per living unit that one could baseline 'cramped' units against 'spacious' units in an area.

Local building code? 600sqft/unit minimum?  Let me know your thoughts!