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How to buy a carry on bag 
Look up your airline's size restrictions. 
Most airlines restrict total bag size to 45 inches (114.3 cm). This breaks down to about 22 inches (55 cm) long , 14 inches (35 cm) wide, and 9 inches (22 cm) deep.
If you know what airline you will be flying most frequently, visit their website to see if their restrictions vary from the standard. Some international carriers only allow a length of 20 inches (50 cm), so keep that in mind if you fly a variety of airlines internationally.