Within Db2 databases, the system table SYSIBMADM.TBSP_UTILIZATION stores information about tablespaces, and specifically their sizes. All rows can be added together to arrive at the size for the entire database.
Once this metric is in place, here is a custom alert that can be used to notify you of excessive growth: https://thwack.solarwinds.com/content-exchange/database-performance-analyzer/m/custom-alerts/3453
To create the Custom Metric in DPA, click on Options > Custom Metrics and clik the "Create new custom metric" button. Make the metric look similar to this, and the attached script contains the query: