How to define the min size of array in the json schema

To set the minimum # of item in an array, use the "minItems".

See:

https://datatracker.ietf.org/doc/html/draft-fge-json-schema-validation-00#section-5.3.3

and

http://jsonary.com/documentation/json-schema/?section=keywords/Array%20validation

   {
   "$schema": "http://json-schema.org/draft-04/schema#",
   "title": "Product",
   "description": "A product from Acme's catalog",
   "type": "object",
   "properties": {
      ...
      "tags": {
          "type": "array",
          "items": {
              "type": "string"
          },
          "minItems": 1,
          "maxItems": 4,
          "uniqueItems": true
      }
  },
  "required": ["id", "name", "price"]
  }