Q: I was wondering if you'd done any comparisons on carry-on luggage. I've been doing a fair amount of flying, and that'll probably only increase. I'm looking for some sort of data so that I can find one of those popular roller bags that would fit into every overhead compartment. It's not so much that I'm looking for a brand (although that would be appreciated) but what dimensions do I need to be on the lookout for.

Q: I was wondering if you'd done any comparisons on carry-on luggage. I've been doing a fair amount of flying, and that'll probably only increase. I'm looking for some sort of data so that I can find one of those popular roller bags that would fit into every overhead compartment. It's not so much that I'm looking for a brand (although that would be appreciated) but what dimensions do I need to be on the lookout for.

A: To be safe, the only rolling suitcase size that will pretty much fit in any overhead bin is an 18-inch model. Although 21- and 22-inch models are technically allowed, they sometimes have trouble fitting. Some regional jets have very small overhead bins and even an 18-inch model might have trouble fitting if the overhead bins are full. TravelPro makes an 18-incher that retails for about $150, but there are many manufacturers who sell models for less. When buying a new suitcase, be sure to get your bonus frequent flyer miles by shopping through the airlines' shopping malls. Just google "airfarewatchdog shopping" to see links to all the airlines' malls. You can sometimes get up to 10 frequent flyer miles for each dollar spent through these sources.

Q: It seems that the major U.S. airlines (Delta, American and Continental) will allow you to use their miles to get "free" business class tickets either on their own international flights or the flights of European partners, but will allow upgrades from coach to business class only on their "own" flights — i.e. flights of the U.S. airline on which you have earned the miles.

This must have something to do with negotiated arrangements between the U.S. airline and the overseas partner. It seems to me that the "partner" airlines would be more willing to give up an upgraded seat which generates some revenue, than a so-called "free" seat, and that correspondingly the U.S. airline would have to "pay" the partner less for the upgraded seat than the "free" seat? So what I am missing about this picture?

A: Well, each airline still operates as its own business with its own business model, so just because they are partners and can offer free seats doesn't mean they have partnered to offer all types of awards. But you're in luck. Some airlines are starting to offer mileage upgrades from economy to business or first class on partner airlines. Delta now offers the opportunity to use miles and/or elite upgrade certificates on partner Air France. US Airways is now offering Star Alliance Upgrade Awards with participating carriers. Check their websites for more information on how to redeem your miles.

George Hobica is founder of the low-airfare listing site Airfarewatchdog.com.