Measuring the width is straight forward noting it at the widest part. All forks have metric equivalents 4" (3.93") or 100 mm, 5"(4.92") or 125 mm, 6" (5.90) or 150 mm
The length is from the tip to the face of the upright.
Thickness is best measured back near the heel (T). If the forks are worn at the heel, then measure the thickness of the upright.
The tip varies particularly on long thick forks. If you feel you have something unusual, let us know.