A lot smaller than it shortly became: essentially the City and Westminster. Smoky, smelly, dirty, noisy, badly lit, and somewhat haphazardly policed. A lot of civil administration was still in theory undertaken at the parish level, which in a city of this size and population density was not very workable.
There is a really excellent website here, Romantic London, which includes maps and contemporary images and descriptions.
See also the marvellous The Proceedings of the Old Bailey. This has very useful background material on the changing London in which the cases tried at the Old Bailey occurred: particularly relevant London,1760-1815, Policing in London.
I also observe that the contemporary resource, Pierce Egan’s Real Life In London, Volumes I. and II. Or, The Rambles And Adventures Of Bob Tallyho, Esq., And His Cousin, The Hon. Tom Dashall, Through The Metropolis; Exhibiting A Living Picture Of Fashionable Characters, Manners, And Amusements In High And Low Life (1821), with its colour plates, is now available by way of Project Gutenberg online and in various downloadable formats.
L.A. Hall, FRHistS