I though the standard beer in Europe was always 500ml.

Nope.

In most restaurants and bars, 300ml is the standard size, sometimes even 250ml. 500ml is considered a big beer. 300ml is also the standard size at festivities, concerts and sports games.

In Colone, the standard size of the local beer (Kölsch) is 200ml, but 300ml is also common.

In the south of Germany, especially bavaria, 500ml is the standard size in restaurants, and 1L at festivities and in bars run by breweries. 500ml is also the standard size of wheat beers nearly everywhere.

In shops, you can get beers in bottles of 330ml and 500ml.

My idea originally comes from the fact that in eastern europe you have either a beer (500ml) or a "small beer" (330ml). Cans are 500ml and bottles vary. In france there is a pint (500ml) or a "half beer" (the usual size) which is 200-250ml. Cans are again 500ml but bottles are all over the place and mostly 330ml.

It depends a little, but in southern Germany (and parts of Austria) 500ml is absolutely the standard and small ones aren't even available in most restaurants (the more regional, the less likely).

I've never lived in the north, just visited very often, and there it could be different depending on the region, setting. Some also say they feel 500ml is the default, others are more in the 330ml range.

I also want to add that 1 standard drink is something defined by the government / health authorities. This isn't the typical drink Bavarians actually drink. It is used for calculation of blood alcohol level or for medical purposes.

Seems that most beers in Bavaria probably approximately equate to 2 standard drinks, meanwhile a small Pilsener commonly found in the North is about 1 standard drink.

Yes, I know (now). But we don't have this measurement of a "standard drink" at all. An Australian ex-coworker had to explain the term, much to my confusion. By "the standard" I simply meant what you will get if you go to a restaurant (or pub or bar) and order "one beer".

I don't think you can generalize this across Europe.

Cans and bottles have/had "preferred sizes" (hence 330mL, 250mL, 500mL being common), but there's no European standard for how much you get in a bar or restaurant.

In Iceland 500ml used to be the standard size of beers at bars maybe 10 years ago. Now 400ml is the most common size (although outside of Reykjavík 500ml is still common). 330ml is the most common size in restaurants and in the liqueur stores.

In the Netherlands the standard beer bottle is 330ml.

When going to a bar there are several types of glasses available for a beer from the tap ranging from 200ml to 570ml (English pint size). When you order a beer, by default you get a 250ml glass.

UK pint is 568ml, a US pint is less than 500ml (because their fl oz are different, and I think even a different number to a 'pint').

Rest of Europe (and Québec) uses 500ml or a litre metric in my experience, yes.

In the Northwest of Germany the default is either 200ml or 300ml (the latter being approximately the size of a typical bottle of beer). For Pilsener a big beer is 400ml. For Hefeweizen a big beer is usually 500ml.

In the Northwest you won't find beers larger than 500ml (< 1 pint)