A compact in the United States isn't the same thing as a compact in Europe. European cars tend to be smaller than American vehicles. For that reason, it's a good idea to take into account the size of your party and the amount of luggage you'll be carrying if you're planning to rent a car in Europe. You may want to ask for a vehicle one -- or even two -- sizes larger than what you normally rent.
On the other hand, many of the roads in Europe are narrower than streets in America. Some roads date back to medieval times, when the most common vehicle to travel down the lane was a horse-drawn cart. Try to find a balance between a car that will comfortably seat your party and one that will let you navigate through tight streets without causing you too much stress in the process.