Most parameter url will be insufficient on varchar (255). Using text that can accept 65535 bytes will be enough.
TINYTEXT 256 bytes
TEXT 65,535 bytes ~64kb
MEDIUMTEXT 16,777,215 bytes ~16MB
LONGTEXT 4,294,967,295 bytes ~4GB
Edit: But if you use mysql more than 5.0.3, then varchar (255) is higher than 65535, so it is better to use it as varchar (20000) for URLs.
VARCHAR . 0 255 MySQL 5.0.3, 0 65 535 5.0.3 . VARCHAR MySQL 5.0.3 (65 535 , ) set used