Indeed, monitor size has little to do with how accurate you are. That's mouse sensitivity and hand/eye co-ordination. On a bigger monitor (or in this case, a taller monitor) the target will be bigger but the ease at which you can get your cross hair on it is exactly the same. Larger just has the advantage that if you're being sniped from 500m, the sniper is just that little bit easier to see.
Quboid