If the whole label is scaled wrong (tiny in one corner, or blown up past the edges), the DPI setting doesn't match your printer. Most label printers are either 203 DPI or 300 DPI. Go to Settings → Printers → Shipping Label Printer, switch to the other DPI option, save, and print a test label. The PostalPoint test label includes two precisely-sized squares; you can measure them to double-check your DPI setting.
If the size is right but the image starts too low or runs off the top or bottom, adjust the label vertical offset. Also double-check the label width, height, and gap you entered. Measure with a metric ruler to the edges of the label itself, not the backing, and measure the gap as the space between two labels on the roll.
If labels drift further out of position on each print, or the printer feeds blank labels, the label end type is wrong. Use "Gap/Web" for normal die-cut labels, "Black Mark" if your labels have black marks on the back, and "Continuous" for gapless rolls on a printer with an auto-cutter. Many printers also need to run a calibration after you change label stock; check your printer's manual.