Are you using the actual dBase III+ program from Ashton Tate? It was a 16 bit, MS-DOS Character mode application. It would be around 20 years old now.
If so, you could use PRG files to send printer control sequences that would set the type size for that particular model of printer. Once you sent a control seqeunce, any printer output from a .FRM or .PRG file would appear in the size you set with the control sequence until the printer was reset.
Coach me a bit on what you are trying to do. I'm not certain that what you want can be done because of the huge differences between an MS-DOS application and a modern Windows application, but I'll give it my best shot!