What is the name of a Large sea cucumber found in tropical Oriental seas body walls used as food Chinese and Japanese?

The large sea cucumber you're describing is likely the "trepang" or "beche-de-mer".

Here's why:

* Large size: Trepang are indeed large sea cucumbers, often reaching lengths of over a foot.

* Tropical Oriental seas: Trepang are commonly found in the tropical waters of the Indo-Pacific region, which includes the Oriental seas.

* Body walls used for food: The body wall of trepang is the edible part, often dried and processed for use in Chinese and Japanese cuisines.

Note: "Trepang" is the more common name in English, while "beche-de-mer" is a French term that is also widely used. There are several species of trepang, but the most commercially valuable are those from the genus *Holothuria*.