Increasing column from VARCHAR(255) in master/slave setup
In a replication environment, for an ALTER that takes a long time, I recommend this:


On one slave:  Take out of rotation (keep clients from using it); perform the `ALTER; put back into rotation.
Repeat for other slaves.
SET @@session.sql_bin_log = 0 so it won't replicate; do the ALTER; set that back to 1.


That way, Slave users will not be impacted; Master user may be impacted.

With Galera / PXC, you would use "RSU" (Rolling Slave Update) and have zero downtime (but have to do it manually in a similar manner).

You need to upgrade to 5.6.  With it many ALTERs (probably including your case) can use the fast ALGORITHM=INPLACE.