All good replies so far. Unfortunately for me, I'm an engineer by training so I like a little more concrete of an answer. I found that answer in what I consider to be the backpacker's Bible, Colin Fletcher's The Complete Walker
(Amazon) Colin Says Pack Volume (in cu.in.) is roughly described by the following forumla:
Pack Volume (cu.in) = (Body Weight + 30) * SQRT(# of Days) * R * S * T * P
Where
R = your resilience
0.75: Sleeps naked on bedrock, hikes barefoot, eats small animals raw
1.00: Sound sleeper, feet always warm, laughs in the teeth of the wind
1.50: Pleasantly normal
2.00: Cold sleeper, comfort hound, serious photographer or gourmet chef
2.50: Feet of ice, special diet, multiple phobias, needs background music
Or any value in betweenS = Season
4 = Summer
5 = Spring or Fall
8 = Winter
Adjust accordinglyT = Gear type
0.9: Ultralight
1.0: Light
1.2: Average
2.0: Cast-iron and canvasand P = party Size
Solo: 1.5
Two: 1.0
Three: 0.7
Four (or more): 0.6So, for me to go out for a 3-day weekend with my buddy in the Fall, I'd plug something like:
(170+30) * SQRT(3) * 1.25 * 5 * 1.1 * 1 = about 2400 cubic inches, or roughly 40L pack... about right for me and my gear.
I have no idea how to adjust this for use when cycling. I'd rather drop my pack and just ride loops (ala base-camping), or pack my gear on the bike (ala cycle-touring).
Hope this helps.