I'm not sure if this glitch is known, and I don't see how it could improve the time of the speedrun, but it would probably make for a better "playground" tas.
I was playing Super mario on my wii, and I was at 3-4 while big (no flower), I was almost stuck at the corner fighting bowser and I managed to jump and get the axe, but around the same time (within a few frames) bowser had hit me and mario was still flashing as bowser fell, now this is where it gets weird, mario walked towards toad the same way he moves if you hold A+B while coming out of a pipe. On 4-1 I was surprised to find that mario was still big, so I hit the ? block to get a fire flower, BUT IT WAS A MUSHROOM, no palette swap or anything, so I touched it, and mario shrunk! later in the level I hit the next ? with a powerup in it, and it was a flower, so I touched it and then mario had the palette he has when he has a fireflower, but he was still small. I shot a couple fireballs and noticed that for about a frame mario returned to his "big" height. I decided to find out what happens when I get hit, and mario grew back to his "big" height, I don't know what happens when he dies "big" from an enemy or from time.
I hope someone puts this glitch to good use!