i use the patagonia scram pack as my summit/light duty pack. checking the patagonia website though, they don't seem to carry them anymore, and i can't remember the dimensions though 750"^3 sticks in my mind.
i bought this pack because it offered more options to me than a camelbak pack, seemed to be made of more robust materials and has a very slim profile. its good for mtn biking, hiking, backcountry snowboarding (it can be used to carry the board horizontally through the shoulder straps) and is easily stored in a larger pack. it fits any 100oz h20 bladder plus a medical kit and some food, and has some lashing points for a shell or helmet, etc.
it would probably require some internet or local store searching to find this pack, but i recommend it as a good all around light duty workhorse.
one downside is price...i believe i paid somewhere near $70 for this pack, but it was comparable in price to the camelbaks in the same size range and it is well padded, well made, and has a very slim profile. you will not notice it on you unless you really overpack it.