The 1st class IJN subs are surprisingly big. The most common U-Boat, the Type VII, is only 67m long. Even the highly advanced Type XXI "Elektroboote" U-Boats are only 76m long. Pretty much all the USN submarines are ~95m long, and the Royal Navy subs were 60-70m long. When I-401 surrendered, Allied sailors could not believe how big she was.
2nd Class
Small TypeSupply
(Army)MidgetI̶-500
Is Ro-500
Because you are short...KaitenHa (3rd Class)Ro (2nd Class)KouhyouJapan Submarine Classification1st Class
Large TypeI (1st Class)I-401Why can't I be named as I-500?