The size of the index and approximate number of rows is available in the INNODB_TABLESTATS table:
mysql> SELECT * FROM INNODB_TABLESTATS WHERE NAME='employees/employees'\G*************************** 1. row *************************** TABLE_ID: 128 NAME: employees/employees STATS_INITIALIZED: Initialized NUM_ROWS: 299468 CLUST_INDEX_SIZE: 1057 OTHER_INDEX_SIZE: 545 MODIFIED_COUNTER: 0 AUTOINC: 0 REF_COUNT: 11 row in set (0.00 sec)