Find the disk size of specific rows
One way is to look at the sys.dm_db_index_physical_stats output. The min_record_size_in_bytes, max_record_size_in_bytes and avg_record_size_in_bytes will give you the sizes you want.

If you want to see the size of a particular record, another way is to find the physical row location, see SQL Server 2008: New (undocumented) physical row locator function. You can then dump the row physical structure, see Anatomy of a Record.

Finally you can use the product documentation to estimate the size:


Estimate the Size of a Clustered Index
Estimate the Size of a Nonclustered Index
Estimate the Size of a Heap