Hi and welcome to the Chiar forum.
As Kerri mentioned it is not the size of the herniation, but if it is restricting CSF flow, o you need a CINE MRI to see if you have that...and remember you have two tonsils, most times they only give you the length of the longer tonsil. Mine were 6mm and 4mm and I had surgery, also keep in mind I had other issues, such as a partically retroflexed odontoid which made the space where CSF flows smaller, then the tonsils in there.....and the width of the tonsils are never mentioned just the length, so, if your tonsils are 13mm's in length but thin like shoe string licorice they may not affect flow, but if they are wide and short even 2mm's it can cork up the opening to almost no CSF can flow.....
What you need is a true Chiari specialist and not a Dr that will do surgery once you loose use of your limbs or something....ugh....
Do you have copies of your MRI's? I wonder how "normal" your MRI was....and another issue is MRI's taken on different machines and from different angles can produce different results....example if you slice it one way ( imagine an upside down pear) say in thirds, and you only look to the far left or right the tonsil can appeaar to be smaller than it is, but look at the middle slice and it can appear to have grown, again depending on how it was viewed the first time....so you need a Dr well experienced and informed on Chiari and ALL related conditions to help you navigate this journey.
We can post a list of Drs for you if you let us know what area you are in, but the list is not a referral nor an endorsement of the Drs listed it is only meant as a starting place to research Drs. Educate yourself on Chiari and ALL related conditions this will help you know when you have found the right Dr for you.
To answer your question, there’s no size of herniation that can cause a syrinx. It’s the instruction to CSF flow that causes that. So it can be a small herniation or a large one.
Also your MRI may not have changed at all. It’s possible that the tech or NL didn’t note it because they think it’s an “incidental finding”. But since it’s congenial I’m betting it was there all along. Perhaps not causing symptoms, but there.
Your symptoms sound like chiari to me and I think a chiari specialist should be your next step. Most drs aren’t as well informed regarding this unfortunately