Looks like T2.medium worked for kjain here:
http://discourse.snowplow.io/t/optimizing-aws-stack-for-medium-load-site/961
One way to test it is to use Snowplow Avalanche (https://github.com/snowplow/avalanche) and do load testing. If you use small to medium instances and put inside an ELB and ASG - you can monitor the logs in cloudwatch dashboard and see if you are hitting capacity or being throttled.