If you are talking about the size of the message you send, even in your mailing list, that is set server wide. IF you are trying to increase it, I wouldn't bother. The recipient server is probably not configured to handle more that 20-25MB message. I believe google, had there message size set to 30MB but lowered it due to the server load..... So sending a message that is too big for anyone to receive is pointless. IT would be better to reduce your message file size. The setting is settings > default >domain default> the click the limits tab, and the you will see mailing list max message size.

Harland, you can use the svcMailListAdmin web service.  There are two functions, GetRequestedListSettings and SetRequestedListSettings, with a setting called maxmessagesize.  Please note that the size is in KBs.