I used the default ebs Ubuntu image from Alestic.com But the volume is only 8GB. Is there any disadvantage?
Depends what you're doing with it. Planning on keeping more than 6GiB of logs?
The larger the volume, the higher the failure rate. Although I've not tested it myself, rumor has it that 1TiB volumes provide the best performance.
Make sure you snapshot regularly as reliability is linked to dirty blocks.