Axon Flux // A Ruby on Rails Blog

SELECT concat(table_schema,'.',table_name),
concat(round(table_rows/1000000,2),'M') rows,
concat(round(data_length/(1024*1024*1024),2),'G') DATA,
concat(round(index_length/(1024*1024*1024),2),'G') idx,
concat(round((data_length+index_length)/(1024*1024*1024),2),'G') total_size,
round(index_length/data_length,2) idxfrac 
FROM information_schema.TABLES 
ORDER BY data_length+index_length DESC LIMIT 20;

Pretty useful way of seeing how big your datasets are. Thanks to Mike at Backtype for sharing this.