If I recall, the Germans had [at a later stage of the war] something like the Italian manned torpedo. I think they tried to use them against the Remagen Bridge. I’m unaware of any mini-subs [and the U-boats aren’t an arwea of particular expertise for me].
At this stage of the war, the Japs have mini-subs, and the Italians may have the manned torpedo. In 1942, the Brits will develop a manned torpedo [flop] and midget submarines [X-craft, a success] in their efforts to sink TIRPITZ.
For the German mission - destroying merchant shipping in the open ocean - I wouldn’t think a mini-sub would contribute much. U-Boats spent relatively long periods at sea so they had to have at least two watches, preferabley three. They had to carry enough torpedos to make it worthwhile. They needed cooks and other support personel. All that says to me: The bigger the better.
The closest thing I can find is the Type XXIII sub which had a crew of only 14. That’s still three times what they are claiming. Only problem is, this sub type wasn’t launched until April of 1944.