"Sheer immensity" refers to something that is extremely large or vast. It emphasizes the overwhelming size or scale of an object or idea. Full definition
«But the principal difficulty for me is the sheer immensity of the problem: all of the homes are in need of so much help, and what we are actually able to do is only a small part of what is needed. (religion-online.org)
Their subsystems are rich in the sheer immensity of their numbers, the variety of types, and the complexity of interconnectedness between them. (religion-online.org)
Scipio then marvels at the sheer immensity of the stars, and sees also that the earth is now very far away, and that the whole of the Roman Empire appears to be no more than a minuscule point upon its surface. (firstthings.com)