In essence, you should be able to fit just under 3 golf balls into the hole. So, if you have lined up the shot correctly, you shouldn’t be missing.
The question that I assume you’re now asking is why are they that size and who stipulated that it should be a rule?
Well, back in 1829, Musselburgh, Scotland, invented the first ‘hole-cutter’ which allowed for all holes to be cut in the same standardized size, so that each hole was fair. If you would like to see the very first, it is kept in a display at Royal Musselburgh’s clubhouse. Some theorize that the implement was invented using steel drainage piping found in the Musselburgh links. The tool works by inserting the tool into the ground, pulling up compacted earth before being used to fill in the previous hole to keep a smooth green surface.
However, It took a while for this particular size to catch-on around various golf clubs. Royal and Ancient Golf Club of St Andrews (R&A), the only known governing body for golf regulations at the time, decided to make it a ruling officially in 1891. The reasoning behind this exact measurement is not particularly clear throughout history; however, it is known that this is the sizing of the first hole cutter. Perhaps, they decided that the first-ever tool to cut a hole should make somewhat of a legacy.