Sizing kafka connect is tricky mainly because of the variety of connectors that you can run on a connect cluster. Some connectors like the S3 connector can be rather lite weight and take up very little resources. Others, like a JDBC connector can be rather heavy.
The common way to size a connect cluster is to figure 1-5mb/s per connect instance with 4-8 cores and 32Gb/ram. This is very thumb in the wind. Deploy your connectors and scale up the number of tasks until you’ve reached your desired throughput or you’ve reached a bottleneck. Normally the bottle neck, in my experience, is CPU. This is mainly due to the (de)serialization and message format conversion that folks do.
Don’t forget to factor in failover. Most commonly it’s N+1 for failover and HA.