I have noticed in mysql that cache_form.MYD table is 14GB. Why did it grow so big? Is it safe to empty it? and how can I avoid this happening in the future?
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https://drupal.org/project/optimizedb - module that will solve your problem. He can clear the table cache_form by Cron, also perform optimization and more. Versions available for Drupal 6.7.
I suspect that cron isn't running on your site. I am not 100% positive about that particular table, but other cache tables do get stale entries removed when cron runs.
If you need help setting up cron, peruse http://drupal.org/cron