In a regular annotation note, create a stack that looks the way you like. It will take a couple of trial and error attempts since there's no preview. Once you have the look you seek, RMB on the note (without double-clicking) and then choose Edit Text in Window. Copy the raw entry from <STACK> to </STACK> and cancel. Edit your dimension and paste this into the Dimension Text edit field.
Example: <STACK size=0.6 divider=no>a<OVER></STACK>