Here's a series of comparison shots that give a good sense of the relative size of the main modern day airlift airplanes used by the USAF. Series was taken on the flight line at Nellis AFB outside Vegas.
I didn't realize what a monster the C5 was til I did this series. The airplanes are the Captain Sim C-130, UKMIL freeware C-17, newly released Area 51 C-5 and HJG KC-135. Probably this series should also have a C-141 but I didn't have one handy for the shots.
I'm still in awe of how big the C-5 is!!