The sizes are intimately connected to the cable core sizes, the carrying capacity of which is determined by national electrical protocols, such as British Standard 7671.

Aluminium cable lugs - TradeIndia

The current carrying capability of the (properly terminated) lug exceed the current carrying capabilities of the associated cable they are connected to.

Try an internet search for the lug sizes as there is quite a range available, both metric and imperial.

That would be best answered by going to the manufacturers web site & looking at the specifications for the sizes you are working with a simple Google search would find this information

The selection of lug is based on cross-sectional area(mm^2)of the core but actually the diameter of lug/core varies as per type of core-sector shaped,stranded,flexible etc

• If the lug is rated less than its associated cable then the lug will fail before the maximum circuit current is reached, which wouldn't be a very happy situation.
• If the lug is rated the same as or more than its associated cable then the circuit protective device will operate before either the cable or the lug fails, which is an entirely happy situation.

Go figure.

In some countries lugs are sold by its cross sectional area not by rated amperes?.

Serach the net for Dowells Cable Ends