What is the optimal size for an Ubuntu (12.10.1) EBS-backed AMI. I
know the default is 8GBs, If I want to boot as a root device, should I
make it bigger? I cannot seem to find too many use cases of what
There is no such thing, as it's completely dependent on your use case. I typically keep my root (/) partition at the default of 8GB, and then add additional EBS volumes (/var, /opt, /home, etc.) as necessary.
Once I make the AMI how do I use it as a root device (and not use the
Right-click on a EBS root AMI and choose "Launch Instance", or select an EBS root AMI from the Launch Instance dialogue. Eric Hammond (also a SF user) maintains a convenient list of the most up-to-date Ubuntu AMIs for each region on his website. Top of the right sidebar.
What is the standard maintenance for this (snapshots)
This is a very nebulous question, but yes, taking periodic snapshots is likely a good idea. You need to be careful, though. Depending on what software is running on your system, taking snapshots without first flushing data to disk will likely cause the filesystem in the snapshot to be corrupt, requiring recovery/fsck on first boot.
For mysql data partitions, ESH's ec2-consistent-snapshot does a nice job.