I'm not 100% certain, but I suspect Inferno takes both with the (to my knowledge) unreleased Gargant being the biggest ship and the Earth model being the biggest 'structure', though there may be other planet models out there that I've forgotten about.
[Edit] I should note that in my opinion as a FREDder, any time you get over a certain size a ship becomes basically scenery. A Super Star Destroyer, if the SWC intends to include one, is pretty much the biggest you can go and still get some semblence of activity out of a ship, so a ship that is 50+ km long is not going to be very useful for anything other than being in the background and looking menacing.