From Forum Nokia Wiki

ID	TSC000012	Creation date
Platform		Devices
Category	Content	Subcategory	Messaging

Keywords (APIs, classes, methods, functions):

Overview

Is there a limit to the size that an MMS message can be?  Does the number of slides matter?
Is there a list of Nokia MMS-capable devices and what size MMS messages they can accept?

Description

There is no limit to the number of slides in an MMS message.  Nor is the size of any particular part of the message limited.  Rather, the limit is the actual size of the message.  For S60 devices, this limit is currently 100 KB, and for Series 40 devices 45 KB.
The Conformance Document recommends 30 KB ¬— you should conform to this to ensure maximum interoperability with all MMS devices on the market.
These sizes can best be found by consulting the Messaging Characteristics In Nokia GSM Devices document. It lists all Nokia MMS-capable devices and includes a "max MMS msg size" field.
Note: At the time of writing, this size limit means for most devices the size of the content plus headers, but headers rarely take more than even 0.5 KB, in fact even half of that (250 Bytes) should be more than enough to reserve for headers in practice.