Having difficulty changing font size with a slider

I'm not sure what was wrong with your text box, but there was something, because it couldn't be selected in its normal state. I replaced it with a normal text box, and it seems to work now.

I adjusted the slider to values from 1 to 7, to match the number of states. I also put in another text box that shows the value of the slider. You can find it in Insert -> Reference. It is a wonderful tool for troubleshooting anything (like sliders) that uses variables.