Simple answer ... and from one of the maker's no less, so it can't be debated
As far as accuracy goes, any helmet that has a "Mystery Helmet" origin, is actually closer in size to the original. That is of course, unless it is a "recast", of a "recast", of a "recast". In which case, the smaller it gets in the long run
In this case, Marrow_Sun's MS3 would be closer in size to the real Fett Helmet. Although, I cannot speak for how much modification it has undergone whilst in his care, only he could answer that. But that's it in a nutshell.
The MSH2 is larger. As is the original MSH. These are "Fan Sculpts" or "Interpretations", with no actual lineage to a real helmet with the exception of laser measurements taken from an exhibit some years ago. And although "laser measurements" may sound impressive, it is not as much so as it may seem. The MSH series was developed in a time when the original Fett Helmet was thought to be a "Little" larger than than what it actually turned out to be. The MSH2 appears larger in some respects, but is comparable in other respects. In any case, it would be up to the perspective fettster. Some of us have larger heads than others
It should be noted though, that although the "Mystery" based helmets have more of a lineage in size, one has to take into consideration that they are still "Smaller" than the original, marginally, depending on how many generations of casting it's been through. So, it could be argued that both the "Mystery Based" and the MSH series are close to the original in size, from two very different perspectives
Confused yet?
Ok. I promised simple. So here it is ...
Rogue Studios, a member here who obtained a first generation casting of a real studio Fett Helmet, has compared a "Mystery Based" helmet to his casting, and they are VERY similar in size. However, as he has said himself, they have some not so “Subtle” differences between them. The “Mystery Based” helmets have a story behind them. The short of it is, it was casting from a a real fett helmet, and then heavily modified/altered by Don Post Studios in pre-production. These modifications were made of course, before a few copies snuck out the back door

Hence the "Birth" of the "Mystery Helmet". Again, this is the abridged version. For the "Full Story", refer to the "Sticky" thread about it. Oh wait, there isn't one ... but there should be

(Helooo Mods)
So in closing, there really is no simple answer, but I hope it helps none the less