For Rome-Milan, looking at trenitalia, about 3 hours with no change of trains. If you use their website, you need to use the Italian name for the cities, Roma and Milano.

If you choose them, the price varies widely depending on how far out you book.

Depending on size of party, you can pick seats in a group of 4 seats which share knee room, or groups of 2 that are like airline seats.

If you are traveling with medium size luggage (too big for airplane overhead but not steamer trunks), will usually fit overhead on frecciarossa (the highest speed and the highest price) trains. The overhead space on frecciabianca trains is smallish, similar size to an airplane overhead, so unless you pack really light, I would avoid the Bianca.

It may be redundant to say so, but pack as light as possible, then pack lighter than that!