[informix] Index extent size estimation -- Posted by inturi on Saturday, June 27 2009
For an attached index, the database server uses the ratio of the index key size to the row size to calculate an extent size for the index:

Index extent size =
(index_key_size/table_row_size) * table_extent_size

For a detached index, the database server uses the ratio of the index key size (plus some overhead in bytes) to the rowsize to assign an appropriate extent size for the index:

Detached Index extent size =
((index_key_size 13)/table_row_size) * table_extent_size

After created, run a query on the system catalog tables or run oncheck –pe or oncheck -pT mydb:mytab to get the size of indexes