Briefcase size allowances vary by airline, but they typically fall under the category of a "personal item." For major international carriers like Emirates, a standard briefcase should not exceed 45 x 35 x 20 cm (approximately 18 x 14 x 8 inches). For most domestic US carriers, the personal item must fit under the seat in front of you, which generally limits dimensions to around 18 x 14 x 8 inches or 17 x 13 x 9 inches. It is essential to check your specific airline's "personal item" versus "carry-on" rules, as a large briefcase might be classified as a full carry-on bag if it exceeds these compact dimensions.