When asked to change a column width on a very large table , such as 30 million rows and disk storage of 30 GB , these are some of the steps I take.

 USE [MyDB]
GO
/****** Resize Table [dbo.MyReports].HolderName to varchar 64  from varchar 50***/

Alter TABLE MyReports
Alter column [HolderName] [varchar](64))

 On a small table – this might be OK to execute and barely any activity will appear. But if attempting to execute on a large table on a busy Production Server – then some tactics are required.

 Some possible tactics:

1)Execute the code as a SQL server Agent task – this will allow the code ,to run as a background task and scheduled for convenience

2)Set the recovery model to Simple , but as this is a single transaction some logging will take place.

3)Backup , before and after.Use the COPY_ONLY option BACKUP to avoid breaking the backup chain

4)If downtime is acceptable : use BCP to export data , TRUNCATE the table , alter column size, use BCP to import data. Keep logging turned off

Deciding on which tactic to apply will depend on a number of factors , including disk storage availability, downtime , how much you can isolate the database

Ref:Jack Vamvas (http://www.sqlserver-dba.com)