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: 
First verify that you are in fact trying to screw the left pedal into the left crank arm and the right pedal into the right crank arm. The pedals are sometimes marked with a R and L on the end of the threaded section. The left side is the side your left hand is on while riding. There are 2 common size pedal threads. Many children's bikes, some lower cost department store bikes and some folding bikes use 1/2" thread on the pedal . Most others use a 9/16" thread on the pedal. My guess is you bought 9/16" pedals and they are too large to screw in to your crank arms. While they make adapters to use 1/2" pedals in 9/16" holes there is not one for the reverse. As an alternative you can ask your local bike shop to determine if the crank arm has enough material to modify it to the 9/16" thread. This likely more expensive them getting the correct pedals.
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!