I'm currently working on a semi-aquatic humanoid race of aliens, the Soliil. The majority of the race has proportions, anatomy and heights close to that of humans, however, I am working on a much younger subspecies that grows significantly larger than the average Soliil while retaining almost the same proportions, sometimes growing tall enough that they can no longer stand unsupported. Unfortunately I haven't been able to find a consistent answer with the few I found varying between 10ft and 20ft.
They still need to be capable of surviving on land, even if loosing some mobility, and able to swim. Preferably, I'd like them to remain close to the same proportions of their smaller cousins, albeit towering above them, but any minor internal changes that could push their maximum height further would be interesting.