There is no one strict answer on what the parameters should be. Usually, we at eazyBI suggest specifying only the Xmx parameter and doing it gradually. For larger instances, the parameters could be more than a few GB. For example, you might want to set it to 6GB or even 8GB -Xmx8g according to available resources on your Jira server (see documentation here: Child process).
The main reason for the OutOfMemory error is complex reports in accounts with large issue counts. Therefore you or report creators might want to check how well the report is built and if there is room for report optimization.