I’d start with a two-container setup with assets on S3 and CDN in front of the site and the S3 bucket. I’d start with 8GB or 16GB of RAM, perhaps depending on what you’re using right now.
How many pageviews do you get in a day and/or month?
If that’s not enough, you could add a load balancer and more servers running the web container.
How much CPU and RAM are you using now? What platform?