make the control description block a standard size
Create the control description at a standard size, based on the font size, but allow the block to be resized to fit the space on the map.
John O commented
I don't follow the bit about the font size.
"When printed, the description sheet boxes should be square, with a side dimension of between 5mm and 7mm" [IOF]
It would be useful to have something like
Item > Set Line Height
With the user then being able to enter a value in mm
The block could then change size, with the number of columns the descriptions were split into staying the same