That's a large backpack, like 1800 cubic inches.
The only really waterproof cycling backpacks I've run across, that size, are the Ortlieb Pro Messenger Bag and Classic Messenger Bag and the large Seal-Line Urban Backpack sold at REI. I've not used either. Ortliebs are generally top quality bags and I'd trust them. I've heard good things about the Seal-Line too.
That said, there are plenty of other cycling backpacks - messenger type backpacks especially - that will be essentially rain proof, even if they aren't as water proof as the three just mentioned.
And there are smaller Ortliebs and Seal-Lines too. I use an Ortlieb Packman.