It could be, although there is not much point in using a generator in itself (generators are meant to provide input for other API modules, e.g. you could add prop=imageinfo to the URL and get thumbnail URLs and whatnot for each image). If you just want to use it on its own, you can call it as action=query&format=xml&list=search&srprop=size&srsearch=london&srnamespace=6&srlimit=10&srgiprofile=wsum_inclinks&srenablerewrites=1

 

For a minimum size, you'd be  using something like gsrsearch=london%20filewidth%3A%3E800 (ie. "london filewidth:>800", URLencoded).