Cubasis 2 runs fine on the iPad Pro 12.9 but does not provide specific screen size options.
The topic is included in our backlog but unlikely to be changed within one of the next updates.
New features/versions will be announced once updates become available.
Please give Cubasis 2 a try on the super iPad Pro 12.9 (have it in use myself).
Cubasis 2 looks and performs great in my opinon.