Question: What about if you have two gears on the crank. I used a 5/8 open wrench to take out my original pedals on my Schwinn Sierra and the new 9/16 pedals won't go in - they're too big. What size would it need? I'd like to use an adapter.

Answer: 
You can't get an adapter to fit a big pin in a small hole, without pushing the pedals farther apart horizontally. To keep the pedals at he same distance, you'd have to drill out the holes to a larger diameter and then rethread them, which probably isn't practical. Just take the pedals back to the shop for a refund and buy pedals in the correct size (which is apparently 1/2in).
It's 5/8" pedals .good luck finding them. Mid school mongoose and schwinn used this size pedal for a few bikes and some old school bikes. I have a mongoose crank and I can't find these size pedals anywhere!