Hello!
What version of Rpg Maker are you using? I use MZ and what I do for bigger sprites is either make an already big sprite or make a smaller one and increase it's size by % with Aseprite. Then, I add "!$" to the character sheet so MZ reconizes it as a single character. But you can also create a regular character sheet (with 6 characters) using bigger sprites and it will still allow you to use them.
If you're using Rpg Maker 2000 or 2003, then you can only use smaller sprites.