JSON formatting customizes how fields in the SharePoint lists and libraries are displayed. The column JSON formatting doesn't change the data in the list item, it only changes how it’s displayed to users who browse the list.
So, there is no way to adjust the size of image through JSON formatting.
As a workaround, you could use flow to reduce image size.
https://powerusers.microsoft.com/t5/General-Power-Automate/Flow-to-reduce-image-size/td-p/2440883
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