All instances are created from an Amazon Machine Image (AMI). The volume or volumes attached to a new instance are volumes created from the snapshots linked to the AMI, and that size determines the default size. If your instances have 8GB root volumes, that's the size of the root volume snapshot of the AMI.
When creating an instance you can, of course, specify larger (but not smaller) sizes for the volume(s), but that default isn't a "preference" that you can customize -- it's an attribute of the AMI you are using to launch.
You can, of course, create one instance from one of the stock AMIs, but with the volume size you want, then make a new, private, custom AMI from your new instance... and when you launch new machines using your custom AMI, the volumes will automatically be the size you wanted.
So, yes, you can get the behavior you want, but it's done using a mechanism other than what you had in mind.