Quote from Engel's (Engel is his correct name😉) Dive System descriptions:-
"For models, such as Robbe's U47, the BTS combined with an Engel Piston Tank is the solution to convert this kit to a static diving model!"
Max external dimensions of the dive tanks are 76x87mm. This is fixed for all tank variants.
Volume is altered simply by varying the stroke of the piston (length of tank).
Also check out what they include in their own VII-C kit, which at 1/34 is about a foot longer than the Robbe version. Be prepared to dig deep into your piggy bank, and anyone else's you can get your hands on😉🤑
Cheers, Doug 😎