Hi and welcome to the Chiari forum.
The way my Chiari specialist handled MRI's was if it was older then 6 months he wanted a new one to review, once he reviewed them, I would not need another one unless I developed new symptoms.
Do u have a Chiari specialist?
The length of the tonsils is not as important as the width or if it is creating a CSF obstruction.
Have u had other symptoms other then what u mentioned above?
There are related conditions that should be ruled out, Syringomyelia, tethered cord, disk issues,ICP, POTS, scoliosis, ehlers-danlos,sleep apnea,
So to answer regarding ur MRI, not until u develop more symptoms, this one can be used as one to compare to...
Do u have copies of ur MRI? and the report?
If not call the facility that did the study and request the MRI and report so u can get a 2nd opinion.
Always request copies of ALL testing so u have on hand this will make getting opinions easier and faster.
Not all with Chiari are surgical candidates, but if u r, research and locate the best Dr for u, one that is a true Chiari specialist, having the right Dr is key.