No, the park ranger won't stop you if you want to try fitting on a site. However, if you don't you'll be out of luck.
As mentioned, the written site size are 'guidelines'. We've fit in many national parks with our 40' motorhome and Jeep. We've visited our favorite national parks enough times that we know where we'd fit or not.
Full-timed for 16 Years . . . Back in S&B Again
Traveled 8 yr in a 2004 Newmar Dutch Star 40' Diesel
& 8 yr in a 33' Travel Supreme 5th wheel