I'm sorry I didn't find a good way to change the size of Mario permanently.
I tried to make a variable for each animation (each animation has two frames, both animated gifs) wich is zero in the beginning. When you collide with the mushroom, the value went to 1 and animations changed to 2nd frames in each animation. It was difficult and when I tested it, it didn't work. (when I collided 2nd time on the mushroom, Mario went back small)
But I found another easy way to do it!!!
You can make two Mario actors. Small one and bigger one. When the game begins, the smaller Mario is there. When you collides with the mushroom, the bigger Mario appears and the small Mario disappears. It will look like, the Mario transform to bigger.
I hope this helped
Maybe Makslane or another GE legend knows better way.