The documentation is unclear, but if you look at the source, it is very clear that the size is in bytes. The size parameter defaults to 10MB, so try the default.

Here is the relevant section of code from the Log::Dispatch::FileRotate module's constructor:

    # Size defaults to 10meg in all failure modes, hopefully
    my $ten_meg = 1024*1024*10;
    my $two_gig = 1024*1024*1024*2;
    my $size = $ten_meg;
    $size = $p{size} if defined $p{size};
    $size = $ten_meg unless $size =~ /^\d+$/ && $size < $two_gig && $s
+ize > 0;
    $self->{size} = $size;
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