Changes have been made in Git (master):

commit f28826dc1c41aa7ccf696d061c2de83493b8b377
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Mon Dec 5 21:29:53 2011 -0700

     [mms] Added config option to limit size of linked messages 
(Request #6247).

  imp/config/conf.xml |    3 +++
  imp/docs/CHANGES    |    2 ++
  imp/docs/UPGRADING  |    2 ++
  imp/lib/Compose.php |   13 ++++++++++---
  imp/package.xml     |    1 +
  5 files changed, 18 insertions(+), 3 deletions(-)

http://git.horde.org/horde-git/-/commit/f28826dc1c41aa7ccf696d061c2de83493b8b377

Added to IMP 5.1.

commit f28826dc1c41aa7ccf696d061c2de83493b8b377
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Mon Dec 5 21:29:53 2011 -0700

     [mms] Added config option to limit size of linked messages 
(Request #6247).

  imp/config/conf.xml |    3 +++
  imp/docs/CHANGES    |    2 ++
  imp/docs/UPGRADING  |    2 ++
  imp/lib/Compose.php |   13 ++++++++++---
  imp/package.xml     |    1 +
  5 files changed, 18 insertions(+), 3 deletions(-)

http://git.horde.org/horde-git/-/commit/f28826dc1c41aa7ccf696d061c2de83493b8b377

The reason should be obvious, linked attachments use resources on the 
server storage, embeded attachments on the network. These are two 
completely different use case that you might want to handle with 
different limits. I see the issue with having to check during sending 
instead of uploading though.

Any reason why this should be any different than what we already have 
for attach_size_limit/attach_count_limit?

Obviously, an admin somewhere could conceivably want to tweak this 
value.  However, it adds yet another 1 (or 2, if you wanted to 
configure max linked attachments count also) config option that is 
mostly duplicative of current config options, and there is no 
practical way of checking for this in certain circumstances until send 
time, so error reporting can be troublesome.

Add a separate configuration option for maximum attachment sizes when 
using linked attachments.