I can somewhat believe the weight, given the height. But I don't believe the height. The only thing I can say that remotely defends 6'3 is that humans have been getting taller over time and she is often in zero- or low-gravity settings. So 6'3? Eh, maybe, even if it's exceedingly rare to find women over 6'0, let alone 3 inches over. That's like finding a dude that's 6'8.
But I only somewhat believe the height because pictures I've seen of her zero suit don't really look that muscular. I mean, clearly the suit does most of the work in movement, because it weighs 200 lbs on its own. So it's not like she needs to be buff to use it. I'm sure there are 6'3 women who do weigh 198, but Samus' physique doesn't appear to be nearly THAT muscular.