A setting to adjust the max attachment size allowed in webmail

AFAIK they are the same thing,

our Postfix template sets the following parameter

http://www.postfix.org/postconf.5.html#message_size_limit

We can easily convert that value to a max upload size for Roundcube configuration. IIRC sogo has a similar setting. If the weight of all attachments exceedes the limit a SMTP error is returned to the application.