Question about Outlook Express

How do I increase the send message size on Outlook Microsoft 2013. Up until the end of June I was able to send emails containing 3 photos and 1 video, however now I usually get a post master fail message of some sort saying that the Message size exceeds fixed maximum message size. Can someone please tell me how to increase the message size as it was not a problem a few months ago, I was able to send emails containing 3 photos and 1 video, now I cannot