Progress update. Currently stuck trying to figure out how to corner boost as mini-Mario on a block that is 3 small Marios high. Does anyone know how to do this one?
Edit: Figured it out. You hold jump for 9 frames, let go for 4, then hold it for 2 or more depending on how high you want to jump.
Heres what I know so far:
In order to corner boost as small Mario on a block 3 small Marios high, you press the jump for 4 frames, let go for one frame, then hold jump a couple more times to get the boost (hold the jump longer afterwards for preferred height and airtime). You can see this in World 1-1.
In order to corner boost as large Mario on a block 3 small Marios high, you press the jump for 1 frame, let go for one frame, etc etc (I haven't been able to do it successfully at 48 speed with shell power though)
There is, however, another type of corner boost when you are in a shell. You jump while hiding in the shell at a block overhead. Then, when you are about to pass the block, let go of the dash button. Mario/Luigi would stick his head out and get a small boost. You lose around 4 speed per frame in the shell, but you gain 20 or more speed in the one frame. This is helpful when you have no choice but to hide in the shell. You can see this most in World 5-3 and once in 1-T.
Then, when you are mini-Mario, you can corner boost off a block that is 2 small Marios high (like the one at the end of the secret exit at 5-G). You hold the jump for 3 frames.
Here are the dsm and avi file (avi file is second link)
http://www.megaupload.com/?d=YU2IXXZF
http://www.megaupload.com/?d=HRCLNY5Q