Toddler bed sticker size?

ok so my niece wants a SPECIFIC bed. Not mentioning the character because this person is not well respected and i do NOT approve of it but anyways. back to the point. she wants her bed to be of a certain someone. but she already has a princess bed and there is NO toddler beds with this character. does anyone know how i can custom make the headboard and foot board sticker and simply replace it. maybe the size of the sticker has something to do? im not sure. but i need ideas people!!!!