How can ZPL scale fonts to fit into a specific block?

Hi,

I've designed a label that contains a section at the bottom to hold delivery numbers.  I can't predict how many delivery numbers will be on each label, but I'd like the font to be as big as it can be for only only delivery number and then automatically get progressively smaller as up to twelve delivery numbers at sent through.

I've used other printing technologies that have a scalable option that can do this, but I can't find out how to do it using ZPL.

Any help would be appreciated.