Picking the perfect suitcase
First of all, I think most people make the mistake of packing for a European vacation the same way they would a weekend get-away by car. That is, they throw in a bunch of stuff they think they may need, just in case, planning to haul the suitcase ten feet to their car, hoist it into the trunk and drive off to their hotel, where they hoist it out and roll it along smooth walkways and up ramps and large elevators to their room. This is how I travel on weekend jaunts, too. This does not reflect how you'll be traveling in Europe.
A better test would be to pack your suitcase, carry it down the stairs (if you don't have a two-story house, borrow one), walk two blocks with your suitcase -- going up and down a few curbs along the way -- then, once you're back home again, carry it up the stairs. Are we having fun yet? Now it's time to rethink what you pack and how you pack.
I personally prefer to carry my bag on to the plane, which means sticking to the maximum size allowed by most airlines: 22"x14"x9". But even if you do plan to check your bag, this is not a license to go hog wild on packing. Regardless of whether you plan to carry your bag on or check it and put your fate in the hands of the baggage handlers, you'll need discipline in choosing and packing your suitcase for a European trip.
Rather than laying out everything you think you may need and then finding a suitcase to hold it all, pick your bag first. Choose a bag that you'll be able to lift and maneuver easily. Stick to one main piece, plus another small piece, like a tote bag or day pack (more on that later). Hard-sided bags weigh a ton empty. Soft-sided bags are a lighter choice. Look for rugged, in-line skate type wheels, a handle that extends and retracts smoothly, and a comfortable grip and handle height.
My perfect main bag is a 20" rolling bag like the Hovercraft 19 or the Tarmac 20, both from Eagle Creek; or the Swiss Army WT-20 from Victorinox. I have never had a problem carrying on my 20" bag, even on smaller commuter planes. You won't find these at your local discount department store, and they're not cheap, but my Eagle Creek roll-on has served me well for over 10 years. Considering that I am on the road for 3 to 4 months of each year, that probably equates to a life time of use for most people, which is pretty amazing. I admit to being on the extreme end, but years of experience have taught me that SMALL + LIGHT = ME HAPPY.
For those who want to carry on their luggage, but need just a bit more packing space, there are also 22" configurations like Eagle Creek's Hovercraft 22, or Victorinox's Swiss Army WT-22. A cheaper alternative to these, and the bag that my wife prefers, is Rick Steve's 21" Roll-Aboard. It easily fits into the overhead bin on most airplanes, has all the features of more expensive bags (in-line skate type wheels, smooth handle action and a comfortable height), and that extra inch equates to a lot of space. Hundreds of people have taken my tours, where I limit the suitcase size to 22", so believe me, it can be done. Some of these bags have an expansion feature, which is meant to accommodate some souvenirs you may pick up along the way. If you start out with your bag expanded, you're in trouble . . .
If you go bigger than 22" you'll have no choice but to check your bag, but I still recommend sticking to something 24" or smaller. I've hefted a few of these over the years and, trust me, when fully loaded a 24" suitcase weighs a lot. Swiss Army makes a 24" bag (WT-24), as does Rick Steves. Eagle Creek stretches it a bit more (since you need to check it anyway) with their Tarmac 25. I strongly discourage anyone from taking a bag larger than 25", unless you're traveling in high-style, with door-to-door delivery and bellhops to carry your bags for you.
Next: Organizing the inside . . .
My trusty Eagle Creek bag has bounced over more cobblestones than I can count
Victornix Swiss Army Werks Traveler
Rick Steves' Luggage: road-tested and affordable