Where can I change the size of a single attachment in the incoming mail?
You can't. You can only change the size of a message (an "attachment" is an imaginary thing--it's just more text to the mail server--that clients turn into attachments when they arrive).
That would be set in the Postfix module. You want the General Resource Control page, and the option is "Max size of a message".
However, I would suggest you think long and hard about that. It'll lead to some confused users at some point in the future--sooner or later, depending on how small you set the limit. Postfix already defaults to a pretty reasonable limit (50MB, I think). Probably better to use disk quotas to restrict usage.
Check out the forum guidelines!
There's the value of 10240000, I presume that is in bytes, if I think good this is 100MB. A few days ago friend sent me an e-mail with an attached file, 43MB and got response from the server that the message is too large.
Ok, I know what is it. It's not 100MB but 10MB. Now working great.