17 million node graph is perfectly fine - not large at all for Grakn.
13 million node for instances is therefore also fine.
Now for the 3 Million nodes of ontology ...
a) in terms of size is fine. No one has ever asked this before, but the limit ontology size in Grakn is a 2^31 types.
b) in terms of writing it, though, I don't think you will be writing this by hand? I assume you're going to migrate an OWL ontology? Are you migrating the SNOMED to Grakn?
In any case, the numbers all seem okay to me. I'm happy to discuss further with you, Gabriel.
Let me know how I can help!