In an .htaccess file in the KBPublisher installation directory change and uncomment line:
php_value upload_max_filesize

For example, if the desired filesize is 3 meg, the following directive would be placed in the .htaccess:
php_value upload_max_filesize "3M"

You also may want to change the following directives:

• php_value post_max_size "8M"
  Sets max size of post data allowed. This setting also affects file upload. 
  To upload large files, this value must be larger than upload_max_filesize. Default 8M

• php_value memory_limit "32M"
  If memory limit is enabled by your configure script,  also affects file uploading.
  Generally speaking, memory_limit should be larger than post_max_size. Default 8M

If you are on windows with IIS server .htaccess file will not work for you,
You should update these settings in php.ini file.