Then you really need to redesign the report if you cannot limit the data to fit the space you have. Even if you can calculate the max characters AND the font that would work, you'd have to effect the change in design view, save it, then open the report. Worse, there is only one such control regardless of how many instances of it you see, so I'd bet that the calculated font size would apply to EVERY record, not individually.
EDIT - I see you posted while I was composing. I answered for a report, which is what your original post stated. I see that the example given is for a form, which would present a different problem. In a form, you can navigate from single record to single record and affect a property for only that view. Not so for a report, and I dare say, not for a form whose view is continuous or datasheet.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.