Your carrier can give you a new SIM that is the correct size. You can also cut down the your currently too large SIM. They actually make SIM card cutting tools for this. Lots of videos show people doing it with just scissors, but they do make cheap tools and jigs that make sure it's right.
Usually getting the correct one from your carrier though is only a few bucks, just FYI, and you will still have the same number. They just grab a new SIM off the shelf and program it to your account, no big deal.