I thought, the standard limit is 10 MB. But a good beginning would be a look in the Univention Config Registry. Go to the Univention Management Console (Webbrowser) --> System --> Univention Config Registry. Search for “limit” or horde", there what should be to be found the useful.
Or with CLI:
ucr search limit
sample set parameter for 20 MB:
ucr set mail/messagesizelimit=20480000