Who has to open? It’s on GitHub, likely here: https://github.com/eirannejad/pyRevit/blob/6aa2c493231f4301125f2570dc5efd644691bbad/extensions/pyRevitTools.extension/pyRevit.tab/Analysis.panel/Tools.stack/Inspect.pulldown/ListFamilySizeCreator.pushbutton/ListFamilySizeCreator_script.py#L4
In particular see line 100 and on.