Use something like 7zip to zip the files to compress if you haven't already done that.
You can use the ftp server to upload big files. Upload them to
ftp://ftp.avast.com/incoming - Using Internet Explorer, Connect to the link and drag the file into the Right pane and drop it, that starts the upload, you don't have read access to this folder.
Give the zip file you are uploading a unique name, so they can identify it. It might not be a bad idea to create a text file (readme.txt) with any relevant information, avast topic, user name, etc. etc. in the zip file.