IMO we should simultaneously introduce a new dollar coin and a two-dollar coin. Once you have two-dollar coins, you're able to conveniently make small purchases exclusively using coinage.
Perhaps a more interesting measure is how many coins it takes to buy a Big Mac, or whether the coins are worth so little it would normally be done with paper money.
For example, the Big Mac index of the UK is £3.09, which is fairly reasonable for coins (£2 + £2 + 10p).
For Switzerland, it's 6.50Fr, so 5Fr + 1Fr + 0.5Fr.
But for the US, it's $5.06, which needs the paper $5.
