Do you use it? I thought not. For what it is designed for, it is the perfect size. It is NOT designed to be the only device a user carries. Read the article for how it is supposed to be used. It is a door keeper to advise the user of what is important and what is not. I find that invaluable. I do not user it for surfing the internet, nor do I use it for answering email, although I can in a pinch. I can answer a message with a quick response or even dictate a more complex reply. I can see my upcoming appointments without pulling out my iPhone and invoking the calendar app. . . and I can set new appointments without doing the same. I can answer phone calls, or choose to ignore them without pulling out the phone at a glance of my wrist, knowing that the call is unimportant or can wait until later. That function alone is valuable to someone running a business or even sitting in a play or movie getting an emergency call from a babysitter.
The ability to monitor your health is another function and see it immediately is also a great help.
The calculator watches failed because entry of data required using a pointing device to depress the itty-bitty keyboards. . . and errors were easily made. I had one and I was perpetually starting over with a calculator. Mine allowed inputing formulae. . . and only once or twice was I able to ever get a complete formula input so that it would work.
That is not the case on the Apple Watch. No keyboard. Reading the data on the Apple Watch is easy. It is not the 8x8 LED grid of the numbers of those calculator watches you refer to. . . it is a Retina Display. On my 42mm I can easily discern the details of the 3 point Fonts of second indicators at the twelve hourly locations around the general Utility Watch face at normal reading distances. The whole face is a touch screen.
If there was a "perfect size", it wouldn't need to come in two different sizes.