I am sure an Alwil team member will give the final word, but the format of the temporary file name looks to me as though this is the Webshield buffering http traffic for scanning.
Avast creates temporary files in the avast sub-directory of the system defined Windows temporary file space to buffer all http, nntp and email traffic (if the appropriate provider is active). It then scans the file and finally deletes the temporary file.
This probably represented a file being downloaded as part of http browsing. The file will be as big as it needs to be, according to what you are doing at the time.
Are you using any “streaming” services? It may be that some streaming http activity was being buffered.
Do you have any idea what it may be, possibly according to the file timestamp? (I mean, did you download some big files, or do you listen to some on-line radio… something that may generate a stream of this size?)
That would be probably it (Web Shield is storing the file locally while it is being downloaded). Why the temp file was left there, however, I cannot say… normally, it should be deleted.