Order a Grande in a Venti cup. That way, you'll have plenty of room to add cream, and you may get a little extra free coffee if you catch your barista on a good day.
On the Starbucks app, hit the customize button and scroll to cup options, where you can request a bigger cup. In the store, politely ask your barista for a larger cup.
If a Starbucks employee is nice enough to fulfill your request, say "thank you," but don't expect to get the same service next time, and keep your requests simple.