[header] How to buy a carry on bag [title] Look up your airline's size restrictions. [step] 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. [substeps] 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.