I go by what fits. Different brands fit differently. Many years ago, the sizes were more uniform betweent he brands, and I pretty much avoided purchases that I deemed to be "mis-size-tagged."
I have learned not to buy toddler clothes at a thrift store unless I have her with me. The teen clothes and grade school clothes are usually ok, but many toddler items have alot of shrinkage. My size 2 bebe can fit into size 2, 3 and sometimes 4 from a thrift store. I'm guessing it's due to excessive washing and drying to get all the dirt/food/glue/glitter/fingerpaint/mom's makeup out.