Calculating of the size of nonclustered index rows depends on whether the keys are fixed- or variable-length. Use the first formula if all of the keys are fixed-length. Use the second formula if keys include variable-length columns.
Calculate the size of nonclustered leaf rows with fixed-length keys only:
7 + (Sum of fixed-length keys) = Size of leaf index row
Calculate the size of nonclustered leaf rows with variable-length keys:
9 + (Sum of length of fixed-length keys) + (Sum of length of variable-length keys) (Number of variable-length keys) + 1 = Subtotal
(Subtotal) + ((Subtotal / 256) + 1) = (Size of leaf index row)
The values 7 and 9 in the previous equations are overhead that SQL Server uses for internal storage.