I am not sure by the description whether you have a 5x5 image that you want to only occupy a 3x3 space in game terms or a 3x3 image that you want to have represent a 5x5 creature, but if you want the token to occupy a space in game terms that is different than what is represented by the image, the solution is the same. You can have the token emit a round or square aura that shows the amount of space the token occupies in game terms while allowing the image to be a different size. Here the dragon's image is 5x5 squares. The squares are the default 5 feet by 5 feet. The aura is defined in the unit of measure of the page. So a -5 square aura gives this. N.B. If tokens need to be adjacent to the aura to attack (such as a 5-foot reach in D&D), you will have tokens overlapping each the big token and might need to right click the large token and select "Send to Back" to be able to select smaller tokens again. You can take this the other way as well. A 3x3 image with a 5 aura would give a 5x5 square aura. Essentially, the dragon would occupy a larger area in game terms than what is represented by the image. It's not an API solution, but I am not sure an API solution was necessary here.