There have been only two big Boys made in S. One in S Scale brass by Sunset Models (not Sunset 3rd Rail) back in 1983 and one by Lionel/Flyer in 2008 using the (oversized for S) K-Line Big Boy tooling.
The Sunset Big Boy was a good looking model but ran poorly. It required a lot of tinkering to get them to run well. If memory serves it required at least 36" radius curves.
The ex-K-Line Lionel/Flyer Big Boy was built to about 1:60 proportion instead of 1:64 and is noticeably oversized when placed next to properly scaled 1:64 equipment.
A better solution if your tastes lean toward large steam would be the Lionel/Flyer Challenger (2011,) Lionel/Flyer Y3 (2012,) American Models 4-8-4 (2006-present) or Lionel/Flyer 4-8-4 (2016) :
The American Models 4-8-4's were available as AC conventional Hirail, DC Hirail or Scale. Currently only the scale wheel version is available from AM. The Challenger and Y3 were Lionel Legacy locomotives and the Flyer 4-8-4 can still be found as a FlyerChief (LionChief) controlled. All of these in their Hirail configurations will negotiate the traditional Flyer R20 curves.