This is what MercuryS help says about it. The setting is in the configuration dialog för MercuryS.
ESMTP maximum size If non-zero, the maximum size message MercuryS should accept from connected clients. MercuryS will advertise this via the ESMTP SIZE keyword, and will enforce it on all incoming mail, whether or not it has an ESMTP SIZE declaration. If the connected client uses an ESMTP SIZE declaration, the message will be rejected at the MAIL FROM: stage of the transaction, otherwise it will be rejected after the DATA command has completed. Note that this latter type of rejection may, in a very small number of cases, confuse some very old SMTP clients, but we believe this scenario will be rare enough that you should be able to turn on size restrictions with confidence.
/Rolf