Rick Steves say it best,
“ One Bag – That’s it.”

“The importance of packing light cannot be over-emphasized, but, for your good, I’ll try.

You’ll never meet a traveler who, after five trips, brags: “Every year I pack heavier.

What Size Should You Bring? 
25 pounds (appx)  in a 9″ × 21″ × 14″ carry-on-size bag, backpack or small roller-board.
This will be particularly convenient when you travel overseas.  Rick Steves only allows ONE BAG per traveler. Although this may seem impossible, once you do this you’ll never look back.

When you carry your own luggage, it’s less likely to get lost, broken, or stolen. Quick, last-minute flight changes become simpler. A small bag sits on your lap on the bus or taxi and stashes easily overhead on an airplane. And – you can purchase cheaper tickets due to less luggage.

Packing
How do you fit a whole trip’s worth of luggage into a small backpack or roller-board? The answer is simple: 
Bring very little

Spread out everything you think you might need on the living-room floor. Scrutinize each item. Ask yourself, “Will I really use my snorkel and fins enough to justify carrying them around all summer?” Not “Will I use them?” but “Will I use them enough to feel good about hauling them over the Swiss Alps?” Frugal as I may be, I’d buy a set in Greece and give them away before I’d carry that extra weight over the Alps.