Confluence appears to ignore Tomcat Datasource DB config for this check. I get the nag screen "Your database connection pool size is not configured. The recommended minimum size is 60" after changing to DS config following these instructions - https://confluence.atlassian.com/conf64/configuring-a-datasource-connection-936511522.html#Configuringadatasourceconnection-3.ConfigurethedatasourceinTomcat
According to  https://confluence.atlassian.com/confkb/startup-check-database-connection-pool-size-960713815.html this shouldn't be the case.

I'll attach server.xml, web.xml and confluence.cfg.xml files. They clearly show maxTotal size is 70 with HTTP maxThreads of 56.

It just looks like the check assumes you have configured c3p0 and is probbaly using the default value for it since it's not configured.

Workaround

Use C3P0.