This is a question we've gotten a lot and are working hard to answer it. The challenge for us is that there are many factors that affect this (the texture of your walls, the thickness of the item, the size of the item). For example, walls that are flat hold the magnastickers better and thus are able to hold more weight. The great part is that MagnaStickers work great on textured walls too but hold slightly less weight.
We are working on a chart that would clearly communicate by wall type the maximum size + the maximum weight. For now, we say that the maximum size for LargeHangups is 24" x 36" and that is to keep things on the conservative side. We don't want someone to buy them and say they don't work as described.