Other than the initial 1966 issue of Australian 50 cent coins, all subsequent 50 cent coins are dodecahedral (12 sided). 50 cent coins minted for general circulation have the following dimensions and construction - Composition is 75% copper and 25% nickel, they have a plain edge, weigh 15.55 grams and are 31.5 mm in diameter.
The 5 cent coin is smaller in diameter than the Two Dollar coin. The Australian 5 cent coin is 19.41 mm in diameter. The Australian Two Dollar coin is 20.62 mm in diameter.
The Australian 1966 50 cent coin was the first and last round Australian 50 cent coin produced by the Royal Australian Mint. With the wisdom of hindsight, this coin was a bit of a blunder on the part of the Royal Australian Mint. It was only very slightly larger in size to the Australian 20 cent coin and it had an 80% silver content at a time when the price of silver was increasing rapidly. It took only three of these coins to get one ounce of silver.
Other than the initial Australian 1966 50 cent coin which was round, all subsequent Australian 50 cent coins from 1969 have been dodecagonal or 12 sided. Because they are of a similar size to the 20 cent coin, they were made 12 sided so that vision impaired people could feel the difference.
The Australian 50 cent coin was issued from 1969 as a 12 sided (dodecagonal) coin. This is to provide a good visual and tactile distinction for vision impaired people who may have trouble with coins of similar size.
A two cent US coin is larger in size than a ten cent US coin.
The round Australian 50 cent coin was only ever issued once, in 1966. Three of these 50 cent coins contained one ounce of silver and, as the price of silver increased, the silver content was worth much more than the face value of the coin. The round 50 cent coin at 31.5 mm was also considered to be too similar in size to the 20 cent coin at 28.52 mm. The replacement 12 sided (dodecahedronal) 50 cent coin first issued in 1969 is still 31.5 mm, but its distinctive shape makes it stand out in appearance and feel from other Australian coins.
The Two Dollar coin was the most recent addition to Australia's circulating coinage and had to fit in with, and be distinguished from, 7 existing types of circulating coin. Circulating coins are no longer made of precious metals, so there is no longer a need to make them to any particular size for any particular metal or denomination. There was also a need to consider the visually impaired. The Two Dollar coin is slightly larger than the 5 cent coin and slightly smaller than the 10 cent coin, and significantly thicker than both.
If by a "mule" coin you mean for example, a 1999 Australian 2 Dollar coin reverse struck with the obverse of a different coin, like a similar size New Zealand coin or, on an Australian 10 cent blank, this is an accident of the minting process and a valuation cannot be anticipated. Examples of your coin have sold on eBay for up to $26 AUD. A reputable coin dealer will be able to give a more accurate valuation based on inspection of your coin.
Dollar coins are larger in size and amount.
All coins are of different size and appearance so that they can be told apart visually and by feel.
No such coin exists, the 1825 cent is known as the Matron Head large cent and is about the size of a half-dollar not the size of modern day cents. The Indian Head cent wouldn't be minted until 1859.
Two different kinds of 3-cent pieces were issued in 1869, a small 14mm silver coin that in good condition may be worth about $300.00 and a larger dime size coin made from nickel, that coin in good condition is about $16.00.