If you would prefer exact measurements, check out the satin bow tutorial or the tulle bow tutorial. As a professional florist, I was usually pressed for time. It was easier to use your arm as an approximate measurement.
Just pulle the satin off the bolt about the length of your arm.
I do the same for the tulle. This is, however, twice the width of the satin ribbon, so I fold it in half.
This gives you a 3" width to work with. Later you will fluff it out to the full width after the bow is completed.
Lay the folded tulle UNDER the satin tulle (against the "wrong side) of the ribbon.
You must keep the two ribbons tightly pinched. Both are rather slippery and if you loosen your grip you'll lose the two ribbons matching up for loop and bow size.