Trying to find the appropriate hiking bag

I am a large guy, at 6'2" and a broad chest i am a huge guy. The problem i have is i feel most backpacks look dumb and extremely small. So I am looking at hiking backpacks online but the problem is I have no idea how large these bags actually are.

I hike maybe 1-2 times a year. What this bag will be for is school and when i ride my bike. I love how hiking bags look and i dont want to use a regular school bag.

I have two bags i am considering and i was hoping someone has experience with either of them. They are both in the 40-42L range.

MEC Spirit 40 (http://www.mec.ca/AST/ShopMEC/HikingCamping/Packs/OvernightPacks/PRD~5024-834/mec-spirit-40-backpack.jsp) or the Black Diamond Speed 40 (http://www.mec.ca/AST/ShopMEC/Climbing/AlpineClimbingPacks/PRD~5023-879/black-diamond-speed-40-backpack-unisex.jsp)

I guess the point of this post is would anyone consider any of those bags grossly too large to use for school/riding on the bus or to use when biking (street biking, down town).

And just for reference, a 30L backpack is way..way to small. I have to take 2 of the largest binders you can get to school and a 30L back at best can only fit one.