size_t is listed as one of the arduino data types at https://www.arduino.cc/reference/en/, but it isn't highlighted as a data type or listed in keywords.txt

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully ran tests with your changes