At one point you could order $1 coins from the mint at face value and with free shipping, and they were really happy when they thought that lots of people were starting to use them. They were less happy when they realized just a few people were purchasing them on credit card with cashback, and just instantly depositing them back at the nearest bank to pay their credit card bill.
26.50 mm (1.043 in) - $1.00
24.26 mm (0.955 in) - $0.25
I am not exactly sure of the reason that the mint is so resistant to making the coins a bit bigger (they used to be).
One of the most important features for cash is that it actually be accepted widely, and if I recall, that is a significant problem for $1 coins. I expect the majority machines that accept cash don't accept them, and trying to use them with a cashier is likely to result in amusement or confusion at best, rejection as a very possible outcome, or even accusations of fraud. That there were few instances where an individual would ever get these in normal activities probably made recognition and use even worse, especially as the instances I cam remember often seemed like attempts to push them inconveniently; I seem to remember that some government machines, I think in post offices, would insist on giving change with enormous numbers of one dollar coins, which would likely generate some resentment for users expecting change that would actually be accepted elsewhere.
It likely doesn't help that the design is rather large, eg, it is wider than a two euro coin and almost as heavy, and that one dollar notes are still being produced. For some reason, the US seems far less willing to be decisive in these changes.
Worse. What wound up happening was that the feds encouraged (probably grant funded, IDK) support for it and the only implementers were other governments and the easiest way to check the box was to make all your mass transit ticket machines and the like spit them out as change despite often times not supporting them as payment so a machine would eat your $20, give you a $2 ticket and spit out 18 items about as useful as Chuck E Cheese tokens.
This has mostly gone away as those machines have mostly switched over to cashless.
The real key is they don’t stop making the dollar bill and force the issue.
But hey the penny is finally dying so who knows?
There are also collector-oriented coins but pretty much none of those are actually intended for use.
Edit: fun fact, there are also $2 bills (but those are way more rare and someone might not believe it's real).
I had someone pocket a Susan B. Anthony $1 coin and put their own money in the register to replace it, but that was because it was a rare coin, not because they thought it was fake.
Meanwhile, in my 25 years of living in the US (NJ, SoCal, and NorCal), I can count on one hand the number of times I've come across them "in the wild".
I started collecting them in 2004 by keeping every one I ran into in person and I now have: 3.
The dollar coin has been the same size since 1979, and the same colour since 2000.