Q: Which "package size" should I use in an ingredient?

The short answer is: what you buy in the super market.
It is not necessary to convert the package size yourself. For example: if you're a bartender and you buy your booze in packages of 6 bottles of alcohol per bottle of let's say 75cl, you should put this into the package field:
Then, in the cocktail recipe itself, you should use the shot size (e.g. 5 cl) and the software will do the recalculation for you.
On a further note, the price you enter, should also be for the full 6x75cl package. So if you buy botvodka for 20 euro for 75cl, you'll probably and up writing something like 120 euro. Then, when you use 5 cl in a cocktail, it will recalculate towards 1,33 euro for 5 cl.
Input of package size is best done in the amount you buy it from your supplier, so you can use it in the upcoming the order management module to order the right amount from your supplier.