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: 
9/16" - 1/2" crank adapters can be found online but do work out quite expensive. 

[Not an endorsement](https://www.amazon.co.uk/dp/B0037N6NCG/ref=cm_sw_r_cp_apa_i_g6pgDbRVVV63E) but an example of what to search for.

I notice the OP asked for adapters but considering the price and the fact with the adapters fitted your pedals will protrude further than normal, my actual suggestion is what others have said above and replace the pedals for 1/2" pedals to fit your crank.
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!