1. It's funny, and I can't explain why, but it's true that "wide knowledge" doesn't sound quite right to my ear, although it appears to be a logical thing to say. You also could say "broad knowledge", "deep knowledge" or indeed "extensive knowledge".
2. Yes, you can have a large quantity of things, in particular if that quantity is measured by something other than its number, thus weight of potatoes, volume of mussels, and so on.