Doing so will trigger Windows Search Indexer to cleanup obsolete artifacts and rebuild a new user catalog for the particular user. On the latest Windows releases (expected on 1809 and above, but subject to change) that has this feature, the registry key above is set to 1 after a catalog is successfully created. An absence of this registry key may indicate that the Windows version is older and lacks this newer feature.
If this key is missing, then it may still be possible to trigger a rebuild by deleting the following folder: C:\Users\<Username>\AppData\Roaming\Microsoft\Search\Data\Applications
NOTE: It is intended, but not yet validated, to keep the SetupCompletedSuccessfully registry key value change (as noted above) to serve this particular purpose for future versions. However, please note that the steps above are for troubleshooting/debugging purposes only (especially the second option for legacy behavior). We reserve the right to change this behavior at any time as necessary.
Note: for database size the following as values to use for estimates. Significant deviation from these values will typically be something to investigate.
Usually, 10-20K per indexed item would be estimated for database size.
For Outlook only data, use the higher side (20k) since they are pure text rich content.