Place the brick your going to be using in this case i'm using a 1x1 brick. If you are going to put this in your project you can just incorporate it into a place that you can reach when your small or normal sized.
Time for the eventing. The first event is onActivate>Player>SetPlayerScale>0.200 (all the way to the left). What this is doing is when you click on the brick (onActivate) it targets the person that clicked on it (Player) and sets your size(SetPlayerScale) to the smallest you can be (0.200).
The second event is the same as the first except you don't move the slider so that it stays at 1.000. All this does is sets your size to normal, or it would but if you look at the events you will see that it is changing you to 0.200 then to 1.000 at the same time. This will be fixed in the next step.
The third event is onActivate>Self>toggleEventEnabled>0 1 . What this does is it when you activate the brick and it reaches this event it turns on or off the listed events in this case event 0 and event 1. When you input the event numbers make sure you have a space between each number. Now all that is left is to click on one of the check marks next to 0 or 1. This will make it a blank square and turn off this event but because we have toggleEventEnabled for 0 and 1 what it is going to do is run down the list starting at 0. Looking at the pictures first it is going to set the player to 0.200 then it looks at the second event but its turned off so it then moves to the third event which turns off the first event then turns on the second. This way every time you click on it it will change your size.