Step 1: Read the sentence carefully and understand its meaning.
The sentence states a universal scientific fact.
Step 2: Universal truths and scientific facts are always expressed in the simple present tense.
Step 3: The subject of the sentence is “The circumference of a circle”, which is singular.
Step 4: In the simple present tense, a singular subject takes a verb ending with “-s”.
Step 5: Therefore, the correct sentence is:
“The circumference of a circle measures 3.14159265 times its diameter no matter how small or large it is.”