I NEED KNOW SIZE OF CHAIN!

I'm not quite sure what you're looking for or why you need it. Can you explain what you need it for?

My understanding is that sizes of objects aren't necessarily fixed, although maybe chains are all the same size. If they have a size, it should be tracked somewhere, so looking for it through DFHack should be possible (I doubt anyone can tell you off the top of their head).

I need chain size for weapon made from one trap component and one chain. I know trap component size, but not know chain size.

There are nicer ways to ask. As far as I can tell chains don't have a size, at least not in the traditional sense (they have no item subtype so there is no [SIZE] token. However, if we look at a trap component made of iron and a chain made of iron and compare their weight we can guess a size. An enormous iron corkscrew weighs 12.56 and has a size of 160, an iron chain has a weight of 39.25, therefore, if SIZE is a unit of volume, the chain would have a size of 500. As a comparison, an iron two-handed sword has a weight of 7.65, which would be a SIZE of 97.45, compared to it's listed size of 90. So obviously it's not exact, and for some reason it's huge, but that's the closest you can get. Of course you could also just use the size of whip as an approximation for a weapon chain, since a weapon made with a chain will probably cut off some of the chain anyway.