That should be easy with some events
Let's say each letter is of same size , about 50 x 50
Each letter has an instance variable scale which can be 0 or 1. When 0 then it should be 50 x 50, when clicked scale var changes to 1 and that letter should be 75 x 75. That is the idea.
Execution would be something like this.
If instance var. scale = 1 and width < 75 set width. Letter.Width + 1 and width. Letter.Height + 1
To scale down do the opposite: If instance var. scale = 0 and width > 50 set width. Letter.Width - 1 and width. Letter.Height - 1
Now you probably only need a for each letter loop to set var. scale = 0 on all letter instances each time you click, and set it to 1 only on the clicked letter. That way on each click all letters will scale up or down depending on their instance variable changes.
You can also add some variables to limit scaling, once it reaches 75 x 75 it stops there and on scale down stops at 50 x 50