I'm from a small city, though did actually live in Saddleworth for a bit so I do kind of know Manchester. It is big enough in my opinion, and besides your idea of going to other nearby towns and cities isn't a bad one. I did my first daygame approach in Manchester actually (well Uppermill), but that was before I was aware of 'game' (sadly packed it in shortly afterwards until I heard about sites like this).
I wouldn't say the city centre is that small, there are a fair few interesting side streets and lots of places to look around, same with Birmingham (don't like that city much but can't deny what it can offer). But I cannot deny that I do like the many different locations to explore in London and that you can move from one area to another.
If nothing else, if you get bored of Manchester, you could always do as I do and basically save up some money and then relocate to somewhere more vibrant, be it London or another city which appeals to you.