How big a suitcase for a 2-month trip?

[pilot]

I will be traveling to New York for 2-3 months at a time to do research several times over the next couple of years, and I'm figuring out how I should carry my stuff (clothes, toiletries, books, a little miscellaneous). I'm tempted to get a 27" Victorinox Werks suitcase that's on sale, but I'm worried that it will get up to the weight limit too easily. Once I arrive I won't be moving around with my luggage very much.

Will the 27" be overkill, and I should get the 24" instead? If I fill the 27" with basically only clothes (and put my books, toiletries, etc., in smaller bags), will I be able to stay under airline weight limits (the bag weighs 13 lbs.)? Should I get 2 medium size suitcases instead of 1 big one? I already have a 22" carryon wheeled bag and some big backpacks, but I need something bigger with wheels. There's also the question of if a spinner would be better...

Thanks, travel gurus!