First Method :-

You can set the E-mail size limit from your WHM.

First login into WHM
Second select “Exim Configuration Editor”  present unde ” Service Configuration” section.
Third scroll down and  click on  “Advanced Editor” optoin.

You will see  the empty box (remember select first empty box) where you can type  (right underneath where you see #!!# cPanel Exim 4 Config), enter in the following:

message_size_limit = 100M

Fourth scroll down to the bottom of that screen and click “Save” option.

Exim configuration will be rebuilt with the new option and your Exim will be restarted.

Second Method :- 

You can set the E-mail size limit from server(root).

1)Login to server as root user,

2)Open the file /etc/exim.conf

3)Put below line

message_size_limit = 100M

4)Save and restart exim service

thats all.