If you are refering to Catia V4, I put a space in the text before the single digit number. The text will be set to the oneside, but looks better than different size balloons.
In V4, Balloon sizes are defined under the DRWSTD function. Their size can be defined as dependent on the text size, or a fixed diameter as you requested. They are stored in the project file and you need write access to change or define them.
