The only other Sainsbury's i believed may be larger than Sydenham was the previous Sava Centre store in Merton, but when they rebranded it as Sainsbury's and sold part of it to M&S a few years back it ended up a lot smaller and the one in Sydenham expanded simultaneously.
Also it may sound plausible to say there are far more larger Tesco stores but in actual fact the size of Sainsbury's in Sydenham could almost definetely give them a run for their money. I think we underestimate its size.
Currently the largest Tesco is in Watford but they are building an eco-friendly one in Woolwich at the moment which once built is supposed to be the largest Tesco ever built.
There's a Tesco in Slough which I would say is rather bigger than Savancentre, a Tesco near Hayes (middx) of a similar size and an Asda on the outskirts of Bristol that makes Slough's Tesco look like a corner shop.
It's Finchley Road I think
I used to go there sometimes when I lived in Kilburn, before locating here
It's huuuge. (Altho I still preferred the Waitrose round the corner. Cut me and I bleed Waitrose)