If you are here to change the heatmap size, the Zoom attribute adjusts the entire heatmap size.
To see your changes, refresh your deck browser screen. You can do this by clicking "sync" or clicking on the "Decks" button at the top left seen in the image below.
WARNING: If you rapidly click different decks, Anki may get stuck re-rendering the heatmap. If you do get stuck, just click "sync" or click the "Decks" button on the top left to refresh the screen. This will usually only happen when you first install the heatmap.
All attributes directly connect to HTML. Search on W3Schools for what different options you can put.
Credit goes to u/Usmlebro and u/22eXy for this idea!