braces? brackets?
 

I need to add braces or brackets in my diagrams (something like a '}' symbol). Does anyone know how I can do this without drawing one by hand? Are there any stencils available for this?

I have used fonts for that purpose and it works quite well...

Take a look at the font family "Hiragino" that offers a few variations, even three characters that fit together on top of each other to create very large sizes: ???

you can have them as only characters in a text box that you can rotate as needed and just play with the font size to achieve what you want...

You may even be able to create a .PNG that would be graphically resizeable, though I haven't tried that...

I am also looking for a way to do callouts where the area being pointed to varies in size. Using a character doesn't work well, since the character's weight also varies with size.

Character Palette. A great feature of OS X. Gotta love it. If you want to use a graphic, do "copy as pdf" in OG so that you don't lose resolution in resizing.