Final answer:
The size of a floor access opening to under-floor spaces has evolved from an older guideline to newer, larger recommended dimensions. Exact specifications are dependent on current building codes.
Step-by-step explanation:
The question relates to the specifications for a floor access opening to under-floor spaces, which is an aspect of building design and construction standards. Although the question does not provide specific building codes, it refers to an older rule of thumb that suggested an opening size of 5 inches side-to-side and 7-8 inches deep. The newer recommendations propose a larger size, which is 8 diameters side-to-side and 15 diameters deep. To determine actual minimum required dimensions, it is essential to consult current building codes or standards, which vary by location and the specific building practices in use.