depends how big is 'big enough' for you. The timbuk2 hemlock is definitely not as big as a hiking bag, but it does carry a whole lot while still providing comfort and pretty good weight distribution on your back. I carried several pieces of 8x30" wood and some tools last weekend in the hemlock on the bike with no discomfort.

anyway, i just dug up what i wrote before on the bag. U can check here for more info.