Recently someone asked me about what size coins I use. Personally I use silver dollar size coins, and I believe you should too. The reason is simple, they are bigger and more visible. There’s really no reason to use anything smaller.
Sure some things are easier with smaller coins, but if you take that argument to cards you’ll see it wrong. Poker sized cards are what 99% of magicians use, however there are bridge size cards which hardly anyone uses for magic. These cards are smaller making a lot of card magic easier with them.
Currently it’s easier than ever to get gimmicked coins custom made and they aren’t that much more expensive than standard gimmicked coins. If you wanted a larger size gimmicked coin you can do that. You’re no longer limited to what’s a stock item.
You owe it to yourself and your audience to use dollar sized coins…Unless you are making an artistic choice to use a smaller coin. For example my coin in bottle routine uses a half dollar, but there is a reason I’m using that coin in the presentation. This also doesn’t apply if you are using a borrowed coin for something like coin in ball of yarn.