Index Space Requirements
I’ve shown you the structure and number of bytes in individual index rows, but you really need to be able to translate that into overall index size. In general, the size of an index is based on the size of the index keys, which determines how many index rows can fit on an index page and the number of rows in the table.
B-Tree Size
When we talk about index size, we usually mean the size of the index tree. The clustered index does include the data, but because you still have the data even if you drop the clustered index, we’re usually just interested in how much additional space the nonleaf levels require. A clustered index’s node levels typically take up very little space. You have one index row for each page of table data, ...
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