Adding brackets to schematic graphic

enter image description here

I have a schematic here that I would to add brackets between the dotted lines to mark distances. However, it is awkward when I try to insert “[” in a text box and scale it up. If I drag the text box, it just makes the box bigger with out making the text larger. If I try to increase the font size it becomes too thick.

Also, how do i decrease the frame size so that it fits more tightly to the actual picture?