Hi and welcome to the Chiari forum.
As AllTheRightShades mentioned above, u will want a true Chiari specialist as there are many Drs that are not well informed or experienced with Chiari or Chiari related conditions.
Some with herniations smaller then urs can have more symptoms and issues then those with a 13mm one...it is not the size of the herniation but if u have a CSF obstruction or other related issues...
My herniations were 4mm and 6 mm...we have 2 tonsils and the Drs tend to give just one of the measurements...I also have a partially retro-flexed odontoid which added to the limited space for CSF to flow...so if the Dr is saying u should have no issues bcuz the herniation is not of a certain length....size is not the issue...how it affects u and ur overall health is.
Look at our list of Chiari drs ,this list was compiled by the members here of Drs they have been to, treated by and liked...the list is not a referral nor an endorsement...it is here for u to use to research Drs....
http://www.medhelp.org/health_pages/list?cid=186