A query to find the size of a table's data and indexes:
SELECT table_name, engine, row_format, table_rows, avg_row_length, (data_length + index_length) / 1024 / 1024 AS total_mb, (data_length) / 1024 / 1024 AS data_mb, (index_length) / 1024 / 1024 AS index_mb FROM information_schema.tables WHERE table_schema = 'foo' AND table_name LIKE 'bar';
This can also be a quick way to find the number of rows in a big innodb table.
Also, check these out.