One of the readers of the hobotraveler.com blog researched international carry-on regulations to find the lowest common denominator for bag dimensions and weights. He found that a bag of 19 x 13 x 8 inches (48 x 34 x 20cm) should be accepted by an airline in the world, though most airlines allow much larger bags of 45-50” in combined dimensions. As for weight:
This is helpful info if you absolutely, positively never want to check a bag. The TB Western Flyer at 18 x 12 x 7” is one of the few bags I know that fits comfortably under the size limits. Some recent discussions at the TB forums extol the Flyer as a bag that fits under nearly any airline seat - real plus given the badly bulging bins on most flights these days!