Hi,
For more information about the statistic level its here : VMware vSphere 5.1
And the impact on the DB size can be estimate check here : VMware vSphere 5.1.
Regarding the performance, thats totally depend on the size of your environnement , the DB performance and the host performance.
But according to this KB : VMware KB: Slow vCenter Server performance after increasing the performance data statistics collection level to more…
VMware strongly recommends you to not run at higher statistical collection levels for prolonged periods of time. Running at higher statistical levels is recommended only for a very short term and for test environments. Also, ensure to engage the local database administrators for any changes from the default settings. Statistical collection levels should never be altered from default in a production environment.
Regards,
Julien.