Two rules before any of the specifics. Lay the thing flat rather than holding it up, and say what you measured rather than what size you think it is. "Chest 52 cm flat across" cannot be argued with. "Large" can.
Garments
Chest or bust — flat, armpit to armpit, and say whether that is across or around. Across is the honest one, because it is what you actually did. Length — highest point of the shoulder straight down to the hem. Shoulder — seam to seam across the back. Sleeve — shoulder seam to cuff. Waist — flat across, and for anything elasticated give it relaxed and stretched.
For a saree, the length and the blouse piece. For anything tailored, the inside leg.
Footwear
The insole, heel to toe, in centimetres. A printed size is a claim about a factory; an insole is a measurement. Give both, and say which country's size is printed.
Furniture and large things
Width, depth, height, in that order, and the one measurement people forget: the largest single dimension, because that is what has to get through a doorway and into a lift.
Seat height for anything you sit on. Note whether it comes apart.
Framed work and flat things
The image, then the frame. Both, labelled. A buyer hanging something cares about the outside; a buyer comparing editions cares about the inside.
Jewellery and small things
Ring size as the inner diameter in millimetres. Chain length end to end with the clasp shut. Stone dimensions rather than a carat claim unless you have the paper for it.
What makes a measurement trusted
A tape in the photograph. It costs nothing, and it turns a number you typed into a number a buyer can check.
Questions people ask
Centimetres or inches?
Centimetres, and add inches if your buyers ask for them. Pick one as the main set and stay with it across your listings.
How exact?
To the nearest half centimetre is plenty. Rounding in your own favour is how returns start.
What if it is worn and sits differently now?
Measure what it is, then say so in the description.
More: writing a listing title, photographing an item, questions to ask before you bid.