1. Large sunspots are usually twice as big as the Earth's diameter. The largest sunspot seen since 1900 is "The Great Sunspot of 1947" which was around 40 times the Earth's diameter.
2. The sunspot activity during the second part of the 17th century was extremely low. This period is known as the Maunder minimum. This coincided with the "Little Ice Age" (1645-1715) and is believed to be responsible for it.
Right: Image showing sunspot activity for the last 400 years. Credits: Robert A. Rohde. http://en.wikipedia.org
3. The sunspot activity is not irregular. The phenomenon has a 11 year cycle of high to low activity. The transition from one cycle to another is not sharp and is marked by the change in overall polarity of the sunspots. The cycle is demonstrated by a famous diagram called "The Butterfly Diagram"
Right Below: The Butterfly diagram. Credits - NASA.
4. Wolf Number: In 1849, Rudolf Wolf introduced the concept of recording sunspot number. This was a very peculiar number taking into account not only individual sun spots but also the number of groups of sunspots. Taking into account sunspot groups reduces the variation due to observation of small sunspots by some observatories. Wolf's number 'R' is given by
R = k(10g + s)
k = observatory factor
g = number of groups of sunspots
s = number of individual sunspots
The Wolf number can be used to denote the solar activity and determine the period of the sunspot cycle.
5. Although sunspots appear dark and are relatively cooler regions on the Sun, they are still as hot as about 4500 K. All known metals would melt at this temperature. They are also a lot brighter or more luminous than the full moon. They appear dark because the surrounding regions of the Sun have a temperature of about 5500 K.