Whenever you're making a sticker, make sure you ask yourself one vital question:
"Where would I put that?"
The size of your stickers is critical to whether or not people can use them. If you're selling a sticker intended for personal planners, a 6" sticker won't work too well. Similarly, if you're selling stickers for fans of a show, a wall-sized sticker won't sell much.
Always make sure you design with the finished product in mind.