Measurements can be in inches or centimetres, so long as you state clearly which. Measure accurately and round up, as steel doesn't stretch!
Measure for collars with the head back to brace the neck muscles. Men should add 1cm or 1/2" for the Adam's apple which tends to be missed by the tape. Be generous. A too loose collar just sits a little lower on the neck, but a too tight collar is unwearable.
Measure wrists with the fist clenched. Measure ankles in standing position. If practical, measure women just before menstruation.