< 3,000 cubic inches - large day pack or small overnight pack for warm weather
3,000 - 3,999 - ideal for three season weekend trip
4,000 - 5,999 - Winter weekend or week in warm weather
6,000 + - Multiweek trek or gear intensive winter
I have a Dana Design Terrplane - 6,300 cubic inches of goodness (and damn it's heavy).