"Due to rider restraint requirements, guests of exceptional size may not be accommodated on some of our attractions. This may apply, but not be limited to, guests who exceed 6'2" or those who exceed 250 pounds (some attractions may have lower weight restrictions), have a 46" waistline or 54" chest."
I don't know about Knotts specificially, I'm going there for the first time in September, but Cedar Point, that's owned by the same company has a similar statement. The trouble with that statement like other's have said, doesn't tell you anything other than you may have problems. And in Cedar points case, each item listed is a different ride.
At Cedar point it reads
Due to rider restraint system requirements, guests of exceptional size may not be accommodated on some of our rides. This may apply, but not be limited to, men who exceed 62, or those who exceed 225 pounds, have a 40 waistline or 52 chest or females who exceed 200 pounds or wear size 18 or larger. Each person has different body proportions so it is not possible to list exact size and weight.
In that case, the 40" waistline is Millinium force that had similar restraints to Xcelerator but both have been modified (in different ways). The 52" chest is Raptor, which is similar to Silver Bullet but Silver bullet has modified "big boy" seats in a couple of rows, so it's 54" at Knotts.
That being said, It also depends on How your big. I was able to Ride Millinium force with a 44" waist because In reality it's more thigh measurement instead of waist. I'm about the OP size 6'4" 280, and the ones I'm most worried about are Xcelerator and Sidewinder.