Change the Sizelim= keyword in the list configuration to the desired amount.   The default for new lists is 30M.

1. Login to Listserv
2. Choose  List Configuration | Edit Manually 
3. Change the Sizelim= setting to the desired size (or add the keyword if not already there)
4. Click Update

See the List Keyword Reference for LISTSERV for more information.

Listserv Video tutorial - How can I restrict the size or type of attachments allowed on my list?