Take a look here if it can help you:
https://meta.discourse.org/t/avatar-size-handlebars-script-no-longer-working/29230/9?u=trash
EDIT: I just see that you are using the script I posted. BTW the old code posted by @Yuun in this topic is still working (on desktop)
https://meta.discourse.org/t/avatar-size-handlebars-script-no-longer-working/29230/5?u=trash