The phrase "unimaginable proportions" means that something is so big, extreme, or intense that it is beyond what can be imagined or comprehended. Full definition
It is surrounded by plenty of farmland (suitable for raising grain and livestock), and it's also at the intersection of several rail lines, so that if we experience an oil spike of unimaginable proportions, you'll still have access to transportation — in fact, the city should thrive as a hub of activity. (businessinsider.com)
unimaginable proportions in order to galvanize efforts at self and social transcendence. (religion-online.org)
Only catastrophe of unimaginable proportions could reverse these developments. (religion-online.org)