Q.How do you size the database for systemcenter reporting database aka MOM 2005 data warehouse ?

Answer: To estimate the prospective Reporting database size, figure out how much your OnePoint database grows within a day. Multiply that number by the number of days worth of data that you want to retain. Then, double that number, to account for indexes.

For example: Let's say you want to retain 1 year worth of data and you have 500 (OML's) managed nodes, and at an average, each one of your managed node spits out approximately 1 MB (this is based on most of the MP's loaded on the MOM management server, your average data may be less and most unlikely to go beyond 1 MB) of data per day.

So, if you do a math based on the explanation above, it would come around something like this:

1 Node in one year will occupy = 1 MB x 365 Days = 365 MB of space

So, 500 nodes in 1 Year will occupy = 365 MB x 500 + 183 GB for Indexes = 366 GB of space.
The TempDB on the SQL Instance containing the OnePoint database needs to be large enough for one days growth.
Also, the SystemCenterReporting database log file should be the size of one days growth, multiplied by five to account for indexes.
Based on the previous example,
TempDB minimum sizing should be = 500 nodes x 1 MB = 500 MB or more
SystemCenterReporting database log minimum should be = 500 MB x 5 = 2.5 GB