Replica volume
For files:
- (Data source size x 3) / 2
For Exchange data:
- Data source size x (1 + log change) / (alert threshold – .05)
For SQL Server data:
- Data source size x (1 + log change) / (alert threshold – .05)
For Windows SharePoint Services data:
- Total size of all databases/ (alert threshold – .05)
For Virtual Server data:
- Data source size x 1.5
For system state:
- (Data source size x 3) / 2
For Hyper-V
- Data source size x 1.5
http://technet.microsoft.com/en-us/library/ff399599.aspx