Most important factors:
Longer arm reduces tracking error.
Shorter arm easier to construct rigidly.
Shorter arm less mass.
The mechanical performance and resonance matching of arm and cartridge are far more important than a tiny reduction in tracking error.
If you don't like tracking error then get a parallel tracking arm.
Very little difference between 9" and 10" arms on any of these factors.
IMO not worth going for 12" but either a 9" or 10" is fine.
Longer arm reduces tracking error.
Shorter arm easier to construct rigidly.
Shorter arm less mass.
The mechanical performance and resonance matching of arm and cartridge are far more important than a tiny reduction in tracking error.
If you don't like tracking error then get a parallel tracking arm.
Very little difference between 9" and 10" arms on any of these factors.
IMO not worth going for 12" but either a 9" or 10" is fine.