If your application is certified (a managed package that's been through the security review), the 10MB limit applies to your own namespace. Your clients still get 10MB for themselves and other, non-certified packages. See Custom Metadata Allocation for more details.
Custom metadata records in certified managed packages that you’ve installed don’t count toward your organization’s allotment. However, custom metadata records that you create do count toward it. This rule applies regardless of whether you create records in your own custom metadata type or in a type from a certified managed package.
In other words, assuming you're planning on going through a security review, your custom metadata will take zero bytes of storage. If not, then it will be the amount listed on the custom metadata type and explained in the documentation above.
Note: this particular note is written from the subscriber's perspective. All you need to know is that records you pre-package in your package won't count against your subscribers, but if they create additional records, those records will count.