Increase the import limit (3MB) for Contact import
The size limit that is an issue is on your webserver php configuration, usually under /etc/php5 on linux

However and by experience, importing a lot of contacts via the web interface is painful. it will time out, you will need to restart and do the mapping of columns->fields, go through the wizard, wonder where it got stuck, see that civi expects "GB" and not "UK" or whatever...

It's much better to write a simple cli script that uses the api to import them, it's going to be easier to run again if needed and way faster.

importing 1.5 million organisations is definitely something you want to do from the cli if you value your time, and care about your mental health.

check out the api explorer, it will generate you the code needed to import a contact

$result = civicrm_api3('Contact', 'create', array(
  'sequential' =&gt; 1,
  'contact_type' =&gt; "Organization",
  'organization_name' =&gt; "bla",
  'api.Address.create' =&gt; array('country' =&gt; "FR", 'city' =&gt; "Paris"),
));


wrap that with your loop reading your csv and you're good to go ;)