how change logs format for apache webdav?

Hi guys, I’ve installed Virtualmin DAV Users 3.5 and works ok, but the current logs are not enough informative for my job, I need get how much bytes are upload/download, the current logs are - - [03/Dec/2015:15:14:48 -0430] “OPTIONS /webdav/ HTTP/1.1” 401 716 “-” “cadaver/0.23.3 neon/0.29.1”

how can I configure the apache logs for include the file change sizes??..thanks!!