The Jupiter III was the Largest space shuttle concept conceived

The Jupiter family of super heavy-lift launch vehicles was part of the DIRECT Shuttle-Derived Launch Vehicle architecture proposed by a group of NASA contract engineers as an alternative to the Ares I and Ares V rockets which were under development for the US Project Constellation.

Research and inspiration source:

An Alternate Approach towards Achieving the New Vision
for Space Exploration

by Stephen Metschan
TeamVision Corporation