Barcode size and hardware limitations
Your thermal printer probably prints at 203 dots per inch (dpi) or 305 dpi. The printer resolution is a hardware limitation on everything it prints, including barcode size. If your printer prints at 203 dpi, the width of barcode lines must be whole increments of 1/203rds of an inch.
For example, a single line of a barcode can be 2/203rds of an inch or 3/203rds of an inch. However, the printer cannot print a line that is 2.6/203rds of an inch.
The eFORMz Composer shows the barcode at the resolution you set. However, your thermal printer cannot accommodate every resolution. ZPL files typically come pre-sized for 203 dpi or 305 dpi and require no adjustment.
If you add barcodes to your project, be sure to test print them, and then scan the test prints with your scanner to make sure the label scans correctly.