Yes, a 90L pack is quite big, and the thing about backpacking is you almost always fill whatever size bag you use. I have a 50L bag that is perfect. In my younger days I did 6 weeks with a 30L (include an SLR), but it really wasn't worth the hassle of packing it so full. The zipper actually broke towards the end of the trip.
I agree that there is such a thing a too small, something the backpacker community seems to eschew [see Tynan article above]. We've been travelling for 6 months with 40L packs, we're consistently wishing they were about 5-10L larger.
However, we make due and are always grateful when we see a fellow backpacker with an enormous bag.
