you probably can, I have never tried it...
if you have an x-2 button next to the font size on the format object, then you can.
You would have to use the length of the field, which isn't always the 'same' due to true type fonts, but it is the best the CR can manage.
Again, I have never tried this, but at times I have thought about it....and then I think about how the report will look with different font sizes.... and I kindof stop thinking about it.
HTH