Hey there,

It is awesome that you're expecting that sort of traffic! There really is not a good way to estimate what sort of droplet size you need for it, because there are so many variables. How efficient is your code? Are you using a database? Which one? How efficient is your data structure? What are the users doing on your site? And the list goes on....

The best way to do this would be to plan from the beginning to scale horizontally. Start with a set of load balancers (https://www.digitalocean.com/community/tutorials/how-to-create-a-high-availability-haproxy-setup-with-corosync-pacemaker-and-floating-ips-on-ubuntu-14-04), setup your application server, then test.

I like to built my application servers small, 1gb droplets usually. Then I test and resize them as needed to handle my load. Because I started with a 1gb droplet, I can scale up and down using the flexible resize to handle load as it comes. Also, droplets created from small snapshots start up faster!

I have an application running at a similar scale to what you are discussing using 2x1gb droplets running haproxy and my application servers (a pool of 10) are all 4gb droplets.

I'd be happy to answer any questions you might have!

Best Regards,

--Andrew