I presume you are using trailing-arm geometry here (same as on a regular motorcycle).
Since this is the rear suspension on a front-drive vehicle, forces imposed by drive torques are not of concern as they are with a motorcycle. So, from this point of view, it can be any length that fits the packaging of the vehicle.
The length of the arm does have an effect during braking, assuming that the braking forces are taken up within the swingarm (usual desogn) and not via a linkage (like BMW). The braking reaction forces will try to pull the swingarm pivot downward, offsetting the natural tendency for the rear to lift up during braking. If you get it right (draw your free-body diagrams) you can have the braking torque reaction very closely offset this, so that the rear moves neither up nor down during braking.
Motorcycle rear suspension nowadays generally uses a linkage to actuate the shock; it's not quite as simple as just mounting the shock/spring above the swingarm. The spring and damper need to be sized and designed to get the correct desired spring and damping rates at the wheel, taking into account any mechanical advantage that may exist through the linkage and/or the way the suspension is mounted.
