Measurements should preferably be taken by another person. Always stand up straight and look straight ahead. BE NATURAL - Relax, and do not inhale and hold your breath or exhale and hold.

CHEST - Measure around the fullest part of the chest (this is generally across the nipple), be sure to keep the tape up under your arms and around shoulder blades. If you will be wearing a thick jumper underneath your jacket, then take the measurements wearing that jumper.
CENTRE BACK - Measured from the nape of the neck to the bottom rear of the jacket (refer to individual product pages for jacket lengths, since length varies by product).

Here is a set of Woodland Leathers sizing charts (not all products are available in all sizes - refer to individual product pages) ...