The size of a twin bed in Japan?

Hi, does anyone know how to find the size of "Twin bed" in a Japanese hotel?

I understand that a "twin size" bed is mostly a "single size" bed in a business hotel in Japan.
But what about the "twin size" bed in a higher end (3.5 or 4 star) hotel in Japan?
(For example, Kobe Portopia hotel (4 start) and Hotel Keihan GRANDE in Osaka (3.5 star))

I'm looking for a room for 2 adult and 1 teenage child (under 17). For the 2 hotels mentioned above, booking.com gives identical rooms whether it is 2 adult or 2 adult+1 child: 2 twin beds. The only difference is the price. It only says that twin size bed may be between 35 to 51in wide.
thanks

In japan anyone over age of 12 is consider a adult, so you need to search for rooms for 3 adults.

Official websites almost always indicate the size of the bed(s).
https://www.portopia.co.jp/en/stay/floor/standard/

110-120cm wide. Enough space for adult and very small child at best. Eg under 2yo.

Your best bet is to book a triple room which is reasonably common in Japan. I've found that a double bed and single bed in the same room is very uncommon unless it's a traditional room where they just lay out more futons on the floor....

kmm899 is right.