Auto sizing of form field in width without changing font size

I am trying to create an adobe letter with form fields in different places of a sentence. Sometimes the data going to the field will only be 1 or 2 characters while other times the data going to the field will be 8 or 10 characters. I want the field to auto size in width without changing the font to auto. The sentence needs to be uniform in font and spacing so that the sentence looks like it is written correctly. How can I create an auto sizing form field?

Below is an example of one of the sentences. I have inserted spaces where I would have form fields.

Building Electricity usage

____________ kWh which is __________of last year’s ____________kWh.  Year to date usage is ____________