The problem is that if you choose too big a geographic area, there will inevitably be a lot of Republicans in that area who will block secession. But if you pick a small area which is overwhelmingly Democratic, like NYC, then almost everyone would be happy if they seceded.
New Yorkers would find out after it's too late that being the financial center of a country that they are not a part of doesn't work too well.
I imagine many of these New Yorkers want the NYC secede to become the new Singapore perhaps? An island of sanity in a sea of lunatic failed big country? Boy, they will quickly discover Singapore succeeds with:
1) A big intolerance of all major and minor crimes. Lee Kwan Yew and his successors make Rudy Giuliani look like John F Kerry.
2) Jesse Jackson, Spike Lee, Al Sharpton, and even half of the New York Times editorial board will quickly land in jail for "inciting racial hatred".
3) There is no free health care, no compulsory social security. If you are not prepared to buy medical insurance or fund your own retirement mutual fund, sorry, the state won't help you and it doesn't care.
4) Public homoexual acts, extra-marital co-habition are both illegal - this alone drives 1/3 of New Yorkers away.
If NYC imagines it can still be like itself if it survives on its own, they will quickly discover they quickly become the rubbish heap of North America (outside Canada of course).