How much do you want to spend and how long would you like it to last? Will you be traveling back to the US frequently and bring loads of stuff each time?
I went with a Consumer Reports suggestion back in the late 90s and bought top end Travelpro stuff. They are still in great shape today despite the many trips and abuse I have dished out by maxing out my weight allowance. (Back when you were allowed 70lbs!) I saved money by finding them on ebay. Although they were a bit pricey, I have saved money in the long run not having to replace something poorly made. I know Consumer Reports rates luggage on a regular basis, and it would be my starting point. If nothing else, it will tell you which ones NOT to buy.
The trains and tubes have gates for people with luggage so don't worry about that point too much.
As for size, I have a huge 26" that only goes transatlantic with me, a 24", a 20" carry on, and a floppy backpacky one for RyanAir.

When I am not using the bigger cases, I store stuff in them. I'd say go as large as you think you can handle. You never know what you might need to haul home one day.
Good luck!
