I prefer giants as heels mostly because I think their size natually leans towards them portraying convincing bad guys or 'monsters'. With that being said however, lets look at a few of the WWE's so called 'giants', most of whom have enjoyed runs as heels and faces.
Kane- debuted as a monster heel, yet he has played the face as well. I believe the Kane character is best suited to his demonic ways when he is wrecking havoc on WWE (much like he is currently doing, only he is a face/tweener as he's doing this in the name of The Undertaker). Perhaps Kane's most successful run as a face was in 2002 when he was arguably Raw's biggest good guy when he went up against HHH. Also in 2002 he was playing the joker, doing Kanearoonies amongst other things - however, as 'un-Kane-like' as this behaviour may have been, I still think it was a necessary progression for the character and in that sense it worked well. Nevertheless, Kane has experienced most of his greatest success as the bad guy in my view, whether it be his original program with The Undertaker or the period following his unmasking...Kane's size, coupled with the monster heel persona is simply a formula that works.
The Undertaker- Again as with Kane, 'Taker has worked as both a face and heel. Many would argue that some of his best work came during the Attitude Era as the evil leader of the Ministry of Darkness. However, the question could be asked, did his success as a heel during this period have to do with him being a 'giant'? For some reason, WWE has never really portrayed him as a true giant as they prefer to promote him based on his supernatural character, yet Kane who isn't that much larger than 'Taker, often is protrayed as a big man (especially in the months following Kane's debut). Whether or not we consider 'Taker a giant in the true sense of the word, I still think the character works well as a heel, yet he is a great face today, perhaps mostly becuase he commands such a respect from the fans.
Big Show- I think this man makes a better heel than he does a face. As a face, he seems one dimensional to me, a face Show often consists of him smiling and shaking the fans hands with not a whole much else there. Yet as a heel, Show often comes across as devastating, powerful and as a legitimate threat to any top face or champion - and I attribute much of this to his overwhelming size.
Great Khali- without doubt this man works best as a heel (and I say that from a fan's persepective). As a heel he was impactful, was given a World title win and was no stranger to the main event. Now as a face, his is a joke and is lucky to make it onto a 2 minute segment (obviosuly this has a lot to do with WWE finally realizing his limited abilities and his knackered knees, but still). WWE probably feel he works well as a face as he gets cheered and long gone are the chants of 'You can't wrestle!' and in that sense it is smart booking from them, but in my own opinion Khali worked better as a heel and again much of this had to do with his massive size...
Mark Henry- again, I just think he is a bland face but as a heel he is much more interesting. It is very easy to believe when you see someone of Henry's size talk smack and then back it up with his brutality and strength, as opposed to being all smiley and light hearted...
Andre the Giant- obviously this guy played both the face and the heel. Admittedly, I think Andre made a good face, he did actually come across as the BFG but again, his size made it so believable when he was a heel...as a fan it's easy to think 'this guy's huge and with his size he is capable of doing bad things to people'. This is why I think big men or 'giants' generally make very competent heels, it just feels natural.
Obviously there are many more to consider but generally I think the bigger you are, the better the chance you will work more effectively as a heel. That isn't to say giants and big men cannot be good faces, it's just that their size naturally appears to lend well to heelish characteristics...