Configuration required for a big mysql database around 20GB
The main configuration settings are in my memory blog.  Most important is 20% of RAM for key_buffer_size.  (However, it does not need to be more than the sum total of Index_length of all the SHOW TABLE STATUS.  This will also do the arithmetic:

SELECT  ENGINE,
    ROUND(SUM(data_length) /1024/1024, 1) AS "Data MB",
    ROUND(SUM(index_length)/1024/1024, 1) AS "Index MB",
    ROUND(SUM(data_length + index_length)/1024/1024, 1) AS "Total MB",
    COUNT(*) "Num Tables"
FROM  INFORMATION_SCHEMA.TABLES
WHERE  table_schema not in ("information_schema", "performance_schema")
GROUP BY  ENGINE;


Beware of a large WP installation -- it uses one database, hence one directory, for each 'user'.  After thousands of users, the OS will bog down because of this.

TokuDB's niche is elsewhere; I would not expect it to be useful for this task.