It varies by the circumstance. If you are serving a table of 6 diners, you should pour out a full bottle divided evenly among the 6 people without requiring the diners to order a second bottle. So those pours would be 125 ml or a smidge over 4 ounces. If there are 5 diners, you would again pour out the whole bottle, resulting in pours of 150 ml or 5 ounces. If there are 4 diners, however, you would pour each diner a 125 ml (4 ounce) pour and reserve the equivalent of 2 more 125 ml pours in the bottle to top off the diners as they draw down their glasses, and when the last of the wine is poured, ask the person who ordered the wine if they would like a second bottle. (OK. Now I'm having flashbacks of my "practical" class where we had to pour 750 ml of water into 4, 5 and 6 glasses such that each pour was exactly even, measured on a digital scale!)
For wines by the glass, pricing and inventory is based on 5 glasses per bottle which is the 150 ml (5 ounce) pour.