This is done on the server in the struts-config.xml file is available in the WEB-INF folder by modifying the segment below:

<!--         Change the maxFileSize parameter below if you want to change the maximum allowed file size for uploaded files.

Valid numbers are for example: 1G (1 gigabyte), 250M  (250 Megabytes), 2048K (2048 Kilobytes)         

More information can be found here: http://struts.apache.org/struts-action/userGuide/configuration.html    --> 

< controller bufferSize='8192' maxFileSize='1G'
processorClass="org.apache.struts.action.RequestProcessor"
contentType="text/html;charset=UTF-8">

Add or modify the above segment below the xml row  "</action-mappings>" in the struts-config.xml file and restart your server