Not so much an answer but something to consider:
Average US male hand is 7.6" across, and their hands are not flat out, so coin is looking roughly 2.2" here, whilst the JW3 coins widely available look a little small ...
Based on that, the coin looks bigger than 1.5", more than 2" if i compared it to my "tiny tim" digits.
Yet, most of the prop coins used that I can easily find are from JW3, and all their coins are 1.5" as @steelersquirrel also found.
Usually a good source, aside from props (persons) themselves, are prop auction houses, such as:
- and they also included plastic stunt coins, and rolls of same said coins, all have the same 1.5" dimensions.
I did notice this when i watched the films, it seemed the coins got smaller. (note: personal perception)
From this I will only say that generally decisions get made at the time of the production that sometimes get lost in time.
So, originally the coin was possibly larger, to make it more..distinctive as this mysterious currency, and then subsequently, long after it was established, they settled on something approaching a more portable (or standardized) size for the rest of the franchise series.
So ultimately, if there was a size change, I think its a production/art direction issue that made the size change...
One thing I came across is that (if) the coins in first film varied in size as a prop, due to art direction, and then when it came to actually producing more of them later (and for the sequels), they could well have reverted to a "Troy ounce" coin (system of weights used for precious metals and gems) standard in weight and size, which in the US this is a Morgan Silver Dollar, weighing a standard 31gr and are 1.5" across.