Allow setting a fixed size of the control description

[fredricj]

Currently the size of the control description is determined by dragging the corners of it.
The orienteering rules instead requires that the boxes be kept squared of a size between 5 and 7 mm when printed. Link to the rules
Please allow setting a fixed size of the definition as this would allow course setters to follow this rule and not waste trying to get the description in the right size.