PHP change the maximum upload file size
En el archivo de configuración de php (php.ini) usualmente ubicado en /etc/php.ini hay que cambiar los valores:
upload_max_filesize
post_max_size
; Maximum allowed size for uploaded files.
upload_max_filesize = 40M
; Must be greater than or equal to upload_max_filesize
post_max_size = 40M
Fuente: http://stackoverflow.com/questions/2184513/php-change-the-maximum-upload-file-size