Infinity,
Looking at the windows driver for your printer I see that it only supports three sizes of the courier font, and these are measured in CPI (not points or pixels). I suspect that, if you want to specify the size in Sesame you will have to use on of the two scalable fonts available: Roman and Sans Serif (probably this is the Arial you are seeing). These can be specified in points, given an even number from 8 to 32. It is likely you will only be able to use the other eight fonts available if you are able to convert the pixels to CPI (characters per inch) and specify a legal size. Even with the two scalable fonts, you will have to try several pixels sizes to find one that matches a point size between 8 and 32 that is an even number.
Additionally, you might try placing a space before the name of the font in the print string call:
Printstring(Purchase_Order_No, 550, 168, 0, " Roman", 15, 0)
Resident Fonts:
Epson Draft 10, 12, 15 cpi
Epson Roman 10, 12, 15 cpi proportional
Epson Sans Serif 10, 12, 15 cpi proportional
Epson Courier 10, 12, 15 cpi
Epson Prestige 10, 12 cpi
Epson Script 10 cpi
Epson OCRB 10 cpi
Epson Script C Proportional
Epson Orator 10 cpi
Epson Orator-S 10 cpi
Epson Roman 10.5 pt; 8-32 pt every 2 pt
Epson Sans Serif 10.5 pt; 8-32 pt every 2 pt
Epson Roman T 10.5 pt; 8-32 pt every 2 pt
Epson Sans Serif H 10.5 pt; 8-32 pt every 2 pt