So I live in West London so our city would lead towards London.
But London is really big and really old with lots of potential places of power. It just seems to big for our town to me, I feel it would lose some of the personal atmosphere. What do you think? Is somewhere like London to big?
Well, maybe.
Take a look at Chicago ... city ~2.8 million; urban ~8.7 mil; metro ~9.8mil and density 12.6k/sq.mi.
London ... city ~7.6 million; urban ~8.3 mil; metro 12-14 mil and density 12.3k/sq.mi.
So actually the urban populations are equivalent and the densities are similar therefore in some ways they map very well. When looking at the Butcher stories most of the action is set in the City of Chicago proper, and even then mostly within limited areas and neighborhoods, but sometimes extends into other parts of Cook County or into surrounding counties. Note that the Chicago Metro area extends 2-3 counties north of Cook County and includes Kenosha County is another state! (Kenosha is in Wisconsin, Chicago is in Illinois)
With London it should be in some ways easier for the players to stay local most of the time but then occasionally foray to other areas for specific tasks/reasons and thus only rarely interact with the local High Council members, go to a more major magic shop, visit a museum for a mission, or such. If you create the city with your players you should have a much better grasp of where the action will most often or most likely take place and then London may not be as overwhelming as you may think (given the players are defining their comfort zone and you have over 2000 years of legends and events to draw from
)
Edit: I was watching the Dresden Files TV series and in there they mention Winnetka (which is still in Cook County, 14mi north of downtown) and even Milwaukee (which is not in the same state and is actually close to the same as the distance between London and Canterbury (according to one site)).
So, when discussing a large metropolis like London it is only too large if you let it become so. You can probably deal with pieces of it well enough, and detail 1-3 areas very well, and then wing it beyond that.