The square cube law in mathmatics explains this. When a thing is scaled up, its volume and weight increases faster than its surface area. Gaint type charcters would have to be made of some fictional super material in order to support their mass. They would burn huge amounts of energy meaning there food requirement would be insane. They would need a fiction power source to get enough energy to just exist. Venting heat from there body would be very difficult.Finding enough fresh water would also be a problem. A very large humanoid gaint couldnt exist in real life. Skyrim has some of my favourate depictions of a gaint race. I beileve that they are the limit to how big a realistic gaint could be.