They describe them pretty often in the books - they are a little larger than palm sized. And they actually seem to add weight to whatever is converted, regardless of the original material.

But, if memory serves, the discussion revolved around three main points:
1) Converting something to cuendillar melds it into a single item, so the harbor chain links became one massive construct rather than articulated links as it originally was. This would make it a problem to create cuendillar armor, since you'd need to convert the links before trying to create mail out of it.
2) The weight issue, which again, seems pretty legitimate in lieu of the fact that if a bunch of warriors are walking around with heavy masses of cuendillar as shields, they are no longer agile to keep up with changing battlefield conditions, and become actually more vulnerable to attack since their movement has been reduced
3) The necessary Talent to mass-produce them seems to depend on a combination of factors that were not easy for the Aes Sedai of the Tower to produce; only a couple of women in the rebel camp had any ability to convert an object to cuendillar without strain and of them, Egwene was by far the mightiest with an affinity for Earth and ability to use the weave in a large scale - thus only one women out of more than a thousand had an ability to produce it in enough amounts to come close to mass production, which seems overly limiting.

Basically, it seems like a great idea on paper, but the properties of cuendillar made it so that even during the Age of Legends, using it for this purpose was not an option, even while they used it for other, smaller things in common use.