I know you didn't recommend a 100. You did however say 50cm should be the minimum height and I think that's just ridiculous. No matter if it is a tank or tub a BP doesn't need 50cm. Yes, the floor space should be sufficient but that was not the question.
My ETB is in a 36x24x24(inches) cage and there is no way my BP would ever need such a cage. Yes, you can certainly provide it but it is not needed or in any way necessary.
Saying "for an adult 50cm height is the minimum if you want to keep it in a proper way" is just flat out wrong. That may be your opinion but having a shorter cage is not improper. You may not agree with it but it's not wrong.