What are the sizes of Japanese double beds? What's the biggest one? I'd like to make a mattress, thanks.

Japanese double beds are divided into "ダブル" and "セミダブル"

「ダブル」is the largest specification. It is about 200X140cm

Kindly suggest you, it is better not to have a mattress on a double bed. It's good to sleep directly on the spring bed. I bought a mattress for my twin bed just like you, and I can't use it at all--

Also, there's a good wool twin mattress for sale on Yahoo in Japan, and I looked at it and saw that it's only 4,900 yen. I think it's a good idea to buy one to save money--