Normally the program determines the font size, while the HORZ: and VERT: options are eventually used for minor adjustments (see Printing.pdf). If the lines are relatively short, you could do something like: LPT1 = HORZ: 12,13,60 VERT: 14,15,50
Sets the margins to 12mm left, 13mm right, 14mm top and 15mm bottom. Sizes the font so 60 characters will fit between the horizontal margins, and 50 lines between the vertical margins. This needs of course some experimenting, so print to PDF first.