Final answer:
Building codes typically specify the dimensions for crawl space access openings, with common sizes ranging from 16" x 24" to 24" x 24" to ensure safety and accessibility. The provided reference to a "rule of thumb" does not appear to directly relate to the question at hand.
Step-by-step explanation:
The question refers to the size requirements of an access opening to a crawl space through a perimeter wall. In construction and building codes, the dimensions of such openings are specified to ensure safety and accessibility for maintenance and inspection. Although the question does not specify a particular code, it's generally understood that building codes may dictate these sizes.
However, the reference provided about the "rule of thumb" regarding sizes does not directly relate to the dimensions of crawl space openings; instead, it seems to provide guidance on a different context, potentially related to spacing in structural elements or foundations. For crawl space access, common dimensions would typically range from 16" x 24" to 24" x 24", allowing enough space for an adult to enter and move within the crawl space for various purposes.