Not sure about for FSX, but for FS9 (some of which should work in FSX)
You have a few options, for the real life stuff, can't get much bigger than the An-225
HOWEVER....
If you don't have to have a 'real life' airplane then may I suggest this mighty beast: (at the bottom of the page)
http://www.simviation.com/simviation/st ... 150&page=5FS2004 EA-Super600X-2 Cargo This aircraft is to my knowledge the largest Sim Cargo aircraft in the Sim world. This aircraft may be demanding on some systems depending on your machine. Be sure to read the notes text document. If you like the challenge of flying large aircraft this is a good one to try. Features= Water Landable/Front And Rear Opening Cargo Doors/Animated= Gear,Pilot, Wheels,Suspension,Controls,Control Surfaces, Spoilers,Leading Edge Flaps/VC/Tinted VC Windows/Mach 1.05 IAS At 30,000/Folding Wings/ Three Paints Chrome S600X-2/U.N./U.S.A.F./ Reverse Thrust Backing/Outboard Pontoons for Water Land/Full lighting/Freedom of Customization/ Model Paint and Concept by Greg Ebnit. 7.9MB
Makes a 747 look like a 737 in comparison.
