String(255) vs String(99999)

When creating MySQL Posts, Will it really matter? I am talking about...I've a lot of hard disk space.

Regrettably, it isn't only a matter of just how much disk space you've.. but it is dependent on the table's limitations/restrictions too:

http://dev.mysql.com/doc/refman/5.0/en/table-size-limit.html

mysql saves the lengthy text fields as varchar when they're short and also the engine carries these to off-table storage when the information is up-to-date to longer size. therefore it does not matter.

if you are using large dimensions you cannot use standard index in your varchar area and you'll have to make use of full text index. myisam supports full text index whereas innodb does not