How Is The Face Value Of A Coin Determined?

As a general rule the 1/2oz silver coins are $10 face value, and 1oz is usually $20. However, as Pacificoin mentioned it's whatever they feel like, for example the $100 for $100 is also a 1oz coin, and the $20 for $20 and the odd $3 coins are both 1/4oz.

When circulation coins were made of silver (5c and above) the size was relative to the value for the most part. The silver 5c was half the weight of the 10c, and a 5th the weight of the 25c etc. While the 5c silver was replaced by a nickel coin the silver coinage retained their relatively relative weight-to-value.

During the 1960's silver was removed from circulation coins and replaced by nickel and copper in both the USA and Canada. The historic sizes were retained because the public was used to it and they would still work in vending machines.

The Canadian half dollar was reduced in size but no one uses it so it really doesn't matter. Dollars in both the US and Canada were reduced in size to what they are now. Since all circulation coins are of base metals now the size and weight doesn't always match any more.

While the "right-sizing" of coins based on value is possible it is unlikely since the public and vending industry would resist and complain too much. I think we will see coins become obsolete altogether before any real changes occur.