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:

As I zoom in, the point symbols remain at the almost the same size:


Is this what you were looking for?
Hope this helps.
Miguel