Limit max upload file size in front end submmission form

You have some ways to do that:

In your functions.php or wp-config.php

@ini_set( 'upload_max_size' , '15M' );
@ini_set( 'post_max_size', '15M');
@ini_set( 'max_execution_time', '300' );

In your .htaccess (if you use apache2)

php_value upload_max_filesize 15M
php_value post_max_size 15M
php_value max_execution_time 300
php_value max_input_time 300

If you use nginx

http {
  client_max_body_size 15m;

In your php.ini

upload_max_filesize = 15M
post_max_size = 15M
max_execution_time = 300

They all work, so you can choose what best suits you, the size is set in MB so all the examples above will set the max upload to 15MB.