Is this puffer down or synthetic?

The material does not state the fill type or fill power. Ask the seller to confirm whether it is down or synthetic before buying if that matters to you.

How warm is a long puffer like this?

A long, hooded puffer with elasticized cuffs traps heat well for everyday winter wear. Without a stated fill rating, treat it as a solid cold-weather coat rather than extreme-cold gear.

Does it run true to size?

The material gives no fit guidance. Puffers are usually worn over layers, so consider sizing up if you plan to wear thick sweaters underneath.

No waterproof or water-resistant rating is given. The matte shell should handle light snow, but it is not described as rainproof.

The jacket itself. No extra hood, bag or accessories are mentioned.

How does it compare with a short puffer?

The long cut covers the hips and upper thighs, which adds warmth and wind protection over a cropped puffer. The trade-off is more bulk and less freedom of movement.