The "informatica.thread.pool.size" is related to number of total connections for executing Tasks/Workflows. This parameter closely related to "informatica.connection.pool.size" parameter. The pool size is for JDBC connections and yes, both parameters should ideally be the same.
The default value is 20 for both parameters. If users intended to run more than 20 jobs concurrently or have long run workflows, then increase the values accordingly.