Generally the middle rows are for people with chest up to like 48-52" The signs will tell you. Some of them, the third seat in every row is meant for larger chest. You'll know the difference because the seat belts will have two buckles rather than one.
I don't know what Manta's restrictions are, and I don't think they have an option for larger seats because of how the ride works. I would also use the test seat that is outside the attraction entrance to see how you would fit.
Big is also a relative term... it can mean different things to different people. So either height/weight or chest size would help a little more in determining if you would fit... or who ever you are asking for.