When you use a silver coin as an investment, the standard is1 troy ounce coin the most economical. You will then get the most silver for your money. If you are buying coins to use as a medium of exchange or means of payment, you can consider buying (even) smaller coins. These are more expensive in comparison, but easier to use as a medium of exchange. A standard silver coin is still worth considering. You then buy the most silver for your money and the coin still represents a low value.
Read more in the Starter's Guide – What do I buy??