Depends on exactly what you mean by 'size'. A simple meaning might just be the number of rows in the table, in which case it'd be
SELECT COUNT(*) FROM someTable;
However, if you likely mean the amount of disk space the tables and indexes are taking up from the tablespace(s), then you want something like
SELECT TABLE_NAME AS `Table`,ROUND((DATA_LENGTH + INDEX_LENGTH) / 1024 / 1024) AS `Size (MB)`
WHERE TABLE_SCHEMA = "whatever"
ORDER BY (DATA_LENGTH + INDEX_LENGTH) DESC;