Since you are not quite sure where to start with, I would recommend to start with General Purpose EC2 instance for production from M category (M3 or M4). You can start with smaller instance type like m3.medium.

Note: If its an internal chat application with low traffic you can even consider T series EC2 instances.

The important part here is not to try to predict the capacity needs. Instead you can start small with general purpose EC2 instance and down the line looking at the resource consumption of EC2 instance you can do a proper capacity planning. Since you can both Scale the instances Horizontally and Vertically, it will require to trade of the instance type also considering Cost and timely load requirements before selecting the scaling unit of EC2 instance.

One of the approach I'm following is as follows

1. Start with General Purpose Instance (Unless I'm confident that there are special needs such as Networking, IO & etc.)
2. Do a load test(Without Autoscaling for a single EC2 instance) of the application by changing the number of users and find out the limits (How many users can a single EC2 instance can handle).
3. After analyzing the Memory, CPU & IO utilization, you can also consider shifting to a different EC2 category or stick with the same type. (Lets say CPU goes to its limit but memory is hardly used, you can consider using C series instances).
4. Scale the EC2 instance vertically by moving to the next size (e.g m3.medium to m3.large) and carry out the load tests to find out its limits.
5. After repeating step, 3 and 4 you can find an optimal balance between Cost and Performance.
   • Lets take 3 instance types with cost as X for the lowest selected (Since increasing the EC2 size in one unit, makes the cost doubles)
     • m3.medium - can serve 100 users, cost X
     • m3.large - can serve 220 users, cost 2X
     • m3.xlarge - can serve 300 users. cost 3X
   • Its an easy choice to select m3.large as the EC2 instance size since it can serve 110 per X cost.
   • However its not straight forward for some applications where you need to decide the instance type based on your average expected load.
6. Setup autoscaling and load balancing to horizontally scale the EC2 instances to handle load above average.

For more details, refer the Architecting for the Cloud: Best Practices whitepaper.