The herniation length isn't the issue. Its the width and shape of them that is imp. If they are blocking csf flow. If there's a syrinx. Drs can't go by the length alone. Has he had spine mri? Cine flow study?
I have to say Ditto to Dana's above post.
Chiari is the malformation of the skull not the herniation itself......it is the result of the skull being to small...it is not the definition of Chiari, but many Drs use it as criteria...which is scary, considering a smaller herniation can cause more issues if it is wider then a very long thin one.....
A CINE MRI is the best way to see if there is an obstruction, and u also have to rule out related conditions like a syrinx , disk issues, sleep apnea,ehlers-danlos, ICP, POTS.....etc....
Severity is determined on how it is affecting overall health and symptoms.