new spinners: check for proper thread first. Spinners should turn quite easily. Alumin(i)um is soft and easily distorted/damaged - so don't force them. Are your hubs new as well? If not, their thread may show some wear or slight damage? Have them checked before doing anything to the spinners.
Longer spinners obviously take less force for same torque - but the torque measurement does not depend on the length of the ears ;-)
On the wide rear wheels (mine are 10x15") the spinner needs to be hit at a slight angle. I use an old rag or strong masking tape to protect the spinner from the blows of the hammer - as J-M says, a spinner adapter and a torque wrench come in handy here.
Best luck - I'm just about to go down to my garage and change rear wheels (hex nuts however)