[exim] accessing message_size_limit within an ACL

Hello list,

I've created an acl, in which I check the size of the data. In the
global sections part I entered

message_size_limit=16M

and in the acl I try:

        deny    message         = Message site $message_size is larger 
than limit of $message_size_limit.
                condition       = ${if 
<{$message_size}{$message_size_limit}{yes}{no}}

but I get an error in the logs:

temporarily rejected after DATA: failed to expand ACL string "${if
<{$message_size}{$message_size_limit}{yes}{no}}": unknown variable name
"message_size_limit"

Is it possible somehow to access the data in message_size_limit?

Rgds,
Gergely Polonkai