I have seen wine poured to various amounts in a wine glass; what amount is generally considered standard...4 oz, 6 oz, 8 oz?
It comes from two different measurements, typical wine bottle size, and government alcohol regulations
Typical wine bottles are 750 ml, and this divides exactly into five or six servings of 150 or 125 ml (thanks peter). So many traditional wine glass serves are "exactly" 150 or 125 ml (~5 or 4 oz) depending in which country you live in
For typical government regulation, a standard drink is; a 100 ml glass of table wine, or a 330 ml can of beer, or a 30 ml glass of straight spirits. Each of these contains on average 10 g of alcohol. So many liquor licensing guidelines recommend the serving of wine by 100 ml (3.5 oz) standard drink sizes
Governments will use this to produce such amazing guidelines such as:
For women: Two standard drinks a day, and no more than ten standard drinks a week For men: Three standard drinks a day, and no more than fifteen standard drinks a week And for both at least two alcohol-free days every week
In most western countries the standard drink range is from 8 to 12 g of alcohol, except in the USA where it is super-sized to 14 g
Update; references from ICAP