Hi and welcome to the Chiari forum.
I know u may not have gotten the info u need to know when or if surgery is needed as u r asking about the size of the herniation.
The size is not as important as if u have a CSF obstruction which it sounds like u do since u had a syrinx form.
A CINE MRI would indicate the obstruction of flow....this in turn can develop into a syrinx...which can cause more symptoms.
The fact u have been DX'd as long as u have with the Chiari could also mean that is how long ur syrinx has been growing.
As for the difference in size of ur herniation that could be bcuz this latest MRI was a different slice (or angle)....and can appear larger or to have grown or shrunk....
Many of us have Chiari and have no idea we have it as it can take a long time to get a DX.....like myself, at ur age a MRI was not available to find it....
Not knowing if ur Drs are Chiari specialists and if they checked u for other related and non related conditions....but do make sure they check u for ALL of them as they can affect how u feel and heal should u have surgery.
Do u need it, most likely since u have a syrinx....I had surgery to help prevent a syrinx from forming.....
Educate urself more on Chiari and research Chiari specialists....as there is a big difference btwn a NS that will do the decompression surgery for Chiari and a true Chiari specialist....do use our list of Drs to start ur research, but know the list is not a referral nor an endorsement....
http://www.medhelp.org/health_pages/list?cid=186