For ascertaining an exact size and fit of your jumper, there will be need to measure only a few precise points of such jumper or sweater that you already do have. Then, you could compare the measurements with your own size chart to be on the ideal size. For flawless measurements, simply put the fabric on a table or smooth surface and then follow these steps.
Simply place the measuring tape at the middle of jumper and start measuring it from back side of collar seam to all the way down to desired hemline.
For this, you will need to position the meter in a horizontal position right down below the seam of your underarm.