I don't know about the online version but the desktop version can handle much more. I have a building that is 228x228 meters.

I think the limit is 100000cm but with a little trickery you can get past that. (Just for testing I once created a terrain of 100kmx100km by editing the values in the Home.xml file. Sweet Home 3D imports and displays that perfectly but you can't zoom or move to the sides because of previous limits.)