Where can I send a larger sample whose avast does not detect?
You can upload files and report to avast here: http://www.avast.com/contact-form.php (change subject to suite Your case)
you can use mail
send to virus@avast.com in a password protected zip file
mail subject: False Positive / undetected sample (select subject according to your case)
zip password: infected
or you can send files from avast chest
how to use the chest. http://www.avast.com/faq.php?article=AVKB21