Final Fantasy Hacktics

Everyone seems to be having trouble using Shishi, including me, but no one will help. I use the 457 Shishi Sprite Editor and all I really want to know is how to "extend my ISO" so that my Sprite that I want to put in will work, because it's too big. Please, just help me with this one thing? By the way, since I know most people don't use it or even know how to work it, I have the PSP version.