As I understand H2 currently doesn't provide the size of index on disk. There is a function disk_space_used but this function doesn't provide index size.

So I suggest to add such posssibility.

As it is more preferable to get index size separately from table size maybe it is better to add function disk_space_used_idx (index_name).

Another way is to provide table and index size (separately from each other) via information_schema. I think that using information_schema will be closer to SQL standard.