Selecting the appropriate oropharyngeal airway for safe and effective airway management is important in clinical practice. In this prospective observational study, we examined the position of the distal end of oropharyngeal airways using a fibreoptic bronchoscope. We enrolled 149 adults (72 men and 77 women). The correct airway size was determined by inserting four adult sizes Guedel airway (Hudson RCI; Teleflex Medical, Research Triangle, Park, NC, USA) (sizes 8, 9, 10 and 11) sequentially in anaesthetised patients. The ‘best fit’ airway was size 10 in 45 (62%) men, and size 9 in 58 (75%) women. However, when these airways were inserted, the distal end of the airway either touched or passed beyond the epiglottis tip in 20 (27%) men and six (8%) women, respectively. When a size-9 airway was inserted in men and a size-8 airway inserted in women, the distal ends were obstructed by the tongue in three (2%) patients. In conclusion, a size-9 airway in men and a size-8 airway in women are the most acceptable sizes for adults of average height.