I think where you want to stay is going to make a big difference as well. With that many people, you may want to look at renting a house. Roman apartments can sometimes claim to sleep more than is really comfortable, especially with couples involved.

Rome is crazy during Easter. We were there (rented an apartment) and took some friends, it was just packed with Italians and we could not show them much because everything was jammed.

One other thing of note, really scrutinize the pictures. If you are a bigger person, those little roman showers are not going to be fun day in and day out. Ditto for washers and dryers, you'll definitely need a washer. Is the furniture covered with sheets or slipcovered? Is there maid service? Do you think you are really going to eat at home? How close are you to a market or supermercati?