Greetings @SamJack
Have a look at the following article by Kristian Ekenes, Autosize by Scale now available in Map Viewer Beta.
I tried this with a point layer representing Peruvian restaurants in London, by enabling the Adjust size automatically checkbox, and I got points with fixed sizes:
rCcQKtXm1B.jpg
As I zoom in, the point symbols remain at the almost the same size:
McCmkgA1lg.jpg
f4NGOssh0o.jpg
Is this what you were looking for?
Hope this helps.
Miguel