What size collar would be best for my chihuahua/min pin mix?

Hey there, I'm looking at buying this collar off amazon and I'm deciding between a size small and a size X-small. Small is 8'-12'' long and 1/2" wide, and the X-small is 6"-10" long and 3/8'' wide... My dog's neck is just a hair over 8" around. She's 6 lbs and she's completely done growing. Thanks!