The file does not know how big the label is
When a file says ^PW812 it is saying 812 dots wide — not four inches. On a 203 dpi printer (8 dots per millimetre) those 812 dots are almost exactly 4 inches. Send the same file to a 300 dpi printer (12 dpmm) and 812 dots are about 2.7 inches.
Which is why the density selector exists in the converter and is not a detail: it supplies the one piece of information the file is missing. There is nothing to infer, because the data is not in there.
- 6 dpmm — 152 dpi. Rare, on older hardware.
- 8 dpmm — 203 dpi. The density of the overwhelming majority of desktop label printers.
- 12 dpmm — 300 dpi. Higher-resolution models, common for small labels and dense barcodes.
- 24 dpmm — 600 dpi. Industrial and precision printing.
How to find your printer's density
If you do not know, 203 dpi is the safe bet: it is the default across Zebra's GC, GK and ZD desktop range and the equivalents from other makes. The model number usually tells you — Zebra's 300 dpi units carry it in the name or the datasheet; without that marking, it is 203.
The definitive answer is the printer's own configuration label, which prints the resolution on it. On most Zebra desktop units that means holding the feed button until the LED flashes the number of times the model's manual specifies.
The test that settles it in thirty seconds
You do not have to find out first. Convert at 203, download the PDF, and read the page size in your PDF viewer: if it says 4 × 6 inches and your label stock is 4 × 6, that was right. If it says something near 2.7 × 4, the file was written for 300 dpi and the selector is the only thing to change.
The PDF from here is always at true physical size, so the number the viewer reports is the answer. Print at 100% scale every time — the print dialog's “fit to page” rescales everything, and it is the second most common reason a barcode stops scanning.