One of my friends attended the Wizarding World of Harry Potter this weekend and aside from the various reports of SHEER JOY she had some news I felt should be shared among anyone planning to attend the park any time soon.

Overweight guests may not be able to ride Harry Potter and the Forbidden Journey. They have some seats out front where you can test to see if you fit and she said her husband couldn't get the shoulder harness over his shoulders. She noted some large-chested women also couldn't get the shoulder harness to work, and that the molded seat would be tight for people with larger thighs.

A little google-fu lead me to a review that includes this snippet.

Speaking of snug seats, it should be noted that a good number of guests are unable to ride Forbidden Journey due to their height and/or girth. There are test seats outside the attraction as well as before you reach the Sorting Hat. While Universal has released the ride’s minimum height requirement of 48″, they haven’t officially stated any maximums. But if you feel like you are taller than most or rather overweight, then there’s a good chance you won’t be able to ride. Fortunately, you can still walk through the queue and enjoy the scenes presented within Hogwarts Castle.
—Spoiler-FILLED Review: Harry Potter and the Forbidden Journey ride at Universal Orlando’s Wizarding World by Ricky Brigante

So, if you're planning to go to Hogwarts and are a bit overweight, you might consider laying off the Honeydukes and seeing if you can get Fluffy to chase you around a bit before heading to the castle.