Illegal? The pixel art police? I don't know if it's illegal but it's highly not recommended to use commercial stuff for any other purpose than study and tests. If you use somebody else's lineart, credit him and as far as 'legality' in this forum is concerned, you're fine. But I don't think you'll learn like this. A lot of people that usually do small pixel art have problems with bigger sizes because the latter is more like real-life sketching and not as dependent on pixel techniques most people can master from the technician standpoint, without need of talent. If you have problems with bigger lineart, getting better at it is probably something this forum can't help you directly with.