The trackpad is indeed too big. I have no idea why they made it so big, I don't think it adds anything on top of the previous size trackpad which was good enough.
Unlike many people, I do not have palm rejection or unwanted pointer movement issues.
I do have other issues though: when typing, I rest my palms on the laptop and because of the trackpad size, they partially cover the trackpad. I can feel the ridge which is unpleasant. This is a minor thing.
The bigger issue is the frequent failure to process 3-finger swipe and drag-and-drop. No idea what causes this. It worked perfectly on my 13" Air, but on the new 15" Pro, I have to very consciously do these movements, since they don't always register, and it's alarming that even if I do them slowly and deliberately, sometimes the laptop simply doesn't perform the functions, sometimes repeatedly. It's frustrating.