There is a common issue whereby a sites 'cache_form' table gets really big. It basically doesn't clear on Cron runs and just keeps growing, often getting to a couple GB in size.
There are various posts about possible causes and cures, but if you find that you aren't able to log into your site to clear your caches due to your DB size limit being breached, then you can use this MySQL query to clear the 'cache_form' table manually. This will just clear the table - it won't break your site and you won't lose any data.
DELETE FROM cache_form
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