I went on a European hotel (superior) tour last summer. Most people have one suitcase and a carry-on handbag or backpack to carry with you on the bus. There is a weight limit on the suitcase (20 kgs or 44 pounds) and on London departures they are very strict about this. If you go over the limit on the day of departure, you will have pay to store some of your stuff for the duration of the trip until you are under the weight limit. There are physical size limitations for your suitcase as well (published in the Contiki brochure), but they are not as strict about those.

However, since they do not weigh your bags after the day of departure, there is no limit on how much you can have at the end of the trip, once you have bought souvenirs and other items.

Can't speak for the luggage on a 14 day tour, but I did an 8 day Getaway to Berlin & Prague last October. I used a wheeled travel bag that I got from RickSteves.com as my check in luggage; it fits easily within Contiki's declared specs on the size of the luggage you're supposed to bring in terms of both dimensions and weight and is even carry-on size, supposed to fit lengthewise into an overhead compartment. As carry-on and for day travel I used a backpack.

Turns out that I was probably the one person with the smallest luggage, everyone else had a larger bag, so they must have found out somehow that Contiki did not strictly enforce their luggage requirements on non-London departure tours. I got along fine just as well, the key was packing not too many clothes, ~1/2 or so of the necessaries, doing laundry as required in the hotel rooms during the longer stays, and not buying too much. For me the purpose of the vacation was to see things and places more than an overseas shopping trip.