we can find or calculate the size of every indexes in a table through sys.indexes and sys.dm_db_partition_stats DMF.
SELECT SCHEMA_NAME(tn.schema_id) [Schema_Name], tn.[name] AS [Table_name],ixn.[name] AS [Index_name], SUM(p.[used_page_count]) * 8/1024.0 AS [Index_size_in_MB] FROM sys.dm_db_partition_stats AS p INNER JOIN sys.indexes AS ixn ON p.[object_id] = ixn.[object_id] AND p.[index_id] = ixn.[index_id] INNER JOIN sys.tables tn ON tn.OBJECT_ID = ixn.object_id GROUP BY tn.[name], ixn.[name], tn.schema_id ORDER BY tn.[name]
Output:-