The template language documentation is at
https://manual.calibre-ebook.com/template_lang.html. It isn't remotely python.
This Template Program Mode template does what I think you want.
Code:
{:'select(formats_sizes(), 'EPUB')'}
You might prefer this one that gives the size in kb or mb:
Code:
{:'human_readable(select(formats_sizes(), 'EPUB'))'}
Or perhaps this one that adds commas to the size:
Code:
{:'format_number(select(formats_sizes(), 'EPUB'), ',d')'}