I have followed the steps for resizing an EC2 volume
Don't have enough rep to comment above; but also note per the comments above that you can corrupt your instance if you start at 1; if you hit 'u' after starting fdisk before you list your partitions with 'p' this will infact give you the correct start number so you don't corrupt your volumes. For centos 6.5 AMI, also as mentioned above 2048 was correct for me.