Essentially you can use VBA code to dynamically change the values. It depends on when you want to format the textbox; when the value is changed or as the default for the form
If for example it is when the value in txt1 is changed
Sub txt1 after_update
me.txt2.fontsize = me.txt1
me.repaint
End Sub.
However, if you change the font size, the size of the textbox will not so your text may not fit into the box.