A Giant is a humanoid larger than the average human being. Not much is known about these creatures, only one is mentioned in Fable, others are seen on pieces of artwork.
Fable & Fable: TLC
The Harbinger is a sword that was once owned by a Hero named Huw the Giant. It is not known if Huw got his title because he was a human of extreme size or that he was actually a Giant. If he was a giant this would also suggest that giants along with humans, would have the ability to be Heroes.
Fable II
A Giant is shown in a piece of concept art for a DLC for Fable II, made by Lionhead's senior concept artist Mike McCarthy. The picture shows a human trying to communicate with a non-hostile-appearing Giant.
Fable III
The same picture described above is shown in the Fable III Artbook under the section "Unused Creatures". This implies that the Giant may have made an appearance in Fable III, but didn't make the final cut.
Fable: the Journey
A silhouette of the Giant described in the picture above is sometimes seen on the titlescreen next to a silhouette of a Troll. This implies that the Giant as well as Trolls might make an appearance in this upcoming title.