The Qmail-Toaster default value is 20971520.
man qmail-smtpd excerpt:
Maximum number of bytes allowed in a message, or 0 for no limit. Default: 0. If a message exceeds this limit, qmail-smtpd returns a permanent error code to the client; in contrast, if the disk is full or qmail-smtpd hits a resource limit, qmail-smtpd returns a temporary error code.
databytes counts bytes as stored on disk, not as transmitted through the network. It does not count the qmail-smtpd Received line, the qmail-queue Received line, or the envelope.
If the environment variable DATABYTES is set, it overrides databytes.