It is not easy to provide a recommendation on the sizing without knowing a lot of details about your application. You can look at some guidelines for right sizing here - https://docs.aws.amazon.com/whitepapers/latest/cost-optimization-right-sizing/tips-for-right-sizing-your-workloads.html
You can also look at using the AWS Computer Optimizer which uses Machine Learning to provide your recommendations for optimization based on observed usage patterns - https://aws.amazon.com/compute-optimizer/getting-started/
Another way you can reduce costs is to sign up for Reserved Instances or EC2 Savings Plans that give you discounts over on-demand pricing based on committed usage
One other suggestion is if your customers are in California deploy in the Oregon us-west-2 region rather than us-east-1 Virginia region.
If t3.large is working very well for you, you can test your workload with t3.medium or t3.small to see if they meet your performance requirements.
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