unfortunately I don't believe you can change text font or size on anything except labels
The simplest way is to just change the field size in the database. You then get a warning in the Designer. There is no simple way to avoid that, but there are several conversations about it over the years on our forums:
http://metastorm.processmapping.com.au/post?id=1468626
http://metastorm.processmapping.com.au/post?id=1469246
http://metastorm.processmapping.com.au/post?id=1574601