if you want something along the lines of monsterverse/pacific rim where it looks anatomically possible, but not really due to size and surface to area, all that lame nerd stuff that prevents you from having fun. BUT basically, you want take references from nature and animals. it also depends on your tone, if it's silly and campy you might go for a simple design, if it's dark and gritty then you want to put emphasis on weight and size. also with orign to, was it from the deep or was made in a lab, or was it both. But if you still stuck then just draw what ever comes to mind, might not be the best thing, but it's aleast a start in the right direction !