Adding brackets to schematic graphic


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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?