The novel doesn't specify the creature's exact height, but it does describe him as being "gigantic" and "hideous". This suggests a large size, but not necessarily 8 feet tall.
The idea of the creature being 8 feet tall likely comes from film adaptations of the novel, which have often portrayed the creature as towering and monstrous.
However, it's important to remember that the novel itself is much more nuanced in its description of the creature. He is, after all, a complex and tragic figure, not simply a mindless monster.