Final answer:
The bur size for a post should not exceed 8 diameters side-to-side and 15 diameters deep, based on more recent research, contrasting with the older guideline of 5 side-to-side and 7-8 deep.
Step-by-step explanation:
The bur for a post should not be greater than a specified multiplier of its diameter. When considering the required size for a bur, an older rule of thumb in dentistry suggested using a bur that is no greater than 5 diameters side-to-side and between 7-8 diameters deep.
However, more recent research advocates for a more considerable bur size, recommending up to 8 diameters side-to-side and 15 diameters deep. This ensures the stability and strength of the post within its placement.