To be able to estimate the total disk space required by aSharePoint 2010 farm, you need to estimate the sizes of the following:
This is the direct attached storage (DAS) required on each server for its operating system, 14 hive, IIS Logs, SharePoint Logs and any other temporary storage. The common values for these are present in my blog article at http://blogs.msdn.com/b/mohamed_el_ashmawi/archive/2011/10/18/sharepoint-2010-capacity-management-confirm-farm-size-after-modelling.aspx
This is the space required for the various SharePoint databases to be used by the SharePoint farm. There are core databases that are present in any deployment such as the configuration database, central administration database and content databases. Moreover, many service applications have their own databases (one or more). An overview of the difference databases and their characteristics is present in http://technet.microsoft.com/en-us/library/cc678868.aspx#Section1
This space is usually present on SAN and is exposed to SQL 2008 clustered instances.
The search index is stored on the query servers and its size depends on the number of items in an index partition. We will go through how to estimate the storage for the index in the article on estimating search requirements.
The SharePoint 2010 configuration database and the central administration database are small in size and each is less than 1 GB.
Estimating storage for content databases can be done using the formula in http://technet.microsoft.com/en-us/library/cc298801.aspx
It is basically: Database size = ((D × V) × S) + (10 KB × (L + (V × D)))
D: Expected number of documents
S: The average size of the documents that you will be storing
L: The number of list items in the environment
V: The approximate number of versions
Another way to estimate the content databases size is to extrapolate from the current SharePoint 2007 (if you are migrating from SharePoint 2007 to SharePoint 2010). To be able to extrapolate correctly, you need to get the size of the current content databases, assume a growth factor for 3 to 5 years; then build the new content database estimate.
Note that it is recommended limiting the size of content databases to 200 GB to ensure system performance.
I will detail the estimates for the service applications databases in the next articles along with capacity estimates.