The printer units are pixels or dots/inch. For example, on a 300 dpi printer, setting the X and Y parameters to 300 would position the barcode one inch from the upper left side of the paper.
The barcode is printed using the resolution of the printer. This means that a barcode that looks correct (in terms of size) on a monitor (e.g. 72 dpi) might print very small depending on the resolution of the printer (e.g. 300 dpi). An easy way to compensate for this is to change the bar size (MakeBarcodeBarSize) and the barcode height (MakeBarcodeHeight) to a larger value when printing the barcode.
The PrintBarcode method is essentially the same as the MakeBarcode method, with different output. See the MakeBarcode method description for details on creating barcodes.