[exim] default message size?

Is there a default message size limit in exim? My exim.conf doesnt have
the message_size_limit setting defined, but seems to be blocking mail
around 8MB. Does this make sense. I've read on the web everything from
10MB to 50MB default limit. And do attachments count towards this
message_size_limit?

The web is a great place to find info, but sometimes hard to find "valid
" info.

Thanks,
Steve