That sucks! I am so sorry to hear that! But I'm going up next weekend (May 2nd) and I'm kindof worried I might not fit either.
I'm a Cedar Point junkie, and I fit in all of their coasters perfectly fine (well okay, Top Thrill and Millennium Force are tight, but hey -- I still fit!). I'm not super huge, but I am plus-size and I always get worried about whether or not I'll fit in the coasters. I understand the weight and height limits, but it's still really embarrassing to think about. Luckily, I haven't been turned away from a seat yet, and I really hope that it doesn't start with Diamondback!
On a side note, it looks like Diamondback's lap harness is very similar to Apollo's Chariot at Busch Gardens. I've rode that one before and was fine, but I didn't know if they were pretty much the same design or if I'm mistaken.
Are there any of the other coasters at Kings Island that are a tight fit? I haven't been since I was in 10th grade, and that was about 8 years ago. I'm still pretty much the same size as I was then, but I dunno about these new coasters