Well, what you're trying to do (a bigger character) is somewhat complicated. You need to design this using assembly code, which is a bunch of code stored inside the Cave Story .exe file.
Framerects, or just rects, are like "slices" of sprite-sheets. To make a big character, you must force the game .exe to "slice" the MyChar spritesheet differently.
Also, how big are the character sprites? Balrog-sized? Bigger than that?
The tool you need is the knowledge of assembly hacking (the stuff that Noxid, GIR, Dubby, Landon, and other people know how to do).
To begin learning, check out version 1.04b of my guide:
http://www.cavestory.org/forums/index.php?showtopic=2314
The guide doesn't actually tell you how to make the character rects bigger, but it does have a general lesson on NPC framerects and how to edit them.
Once you have a basic grasp of what assembly language is, come ask me again and we can start working on it.