Today I was in a little bit of a hurry and didn't check the shirt size before I bought it. I got an L instead of an XL. Would the park be able to do anything if I contacted them? I'm 4 hours away and won't be back soon. I hung onto the receipt. I know I'm most likely SOL. It fits but it's a little tight. 

Your best bet is to call the park. I don't see why an exchange wouldn't be possible, assuming you have the shirt in unworn condition (with all tags, etc) and the receipt. Doing an exchange without returning to the park may be a challenge, as they don't typically ship merchandise.

Let us know what they say.

I just contacted the park. They took my name and number and they said someone from that department will be in touch with me.

My wife did the opposite. Bought a XL when I needed a large. Had no issues swapping with the receipt.

Just a little while ago, I got a call from Robin Igney, the mechandise supervisor who offered to ship me a replacement as soon as I send mine back. I never expected them to do that! A++ customer service from Kings Island!