Hi -- good questions!
Regarding the different sizes, I have two answers. First, there are some limits on how small and how large sunspots can be which have to do with the balance of the pressure of the gas and plasma surrounding a sunspot, and the pressure of the magnetic fields inside a sunspot. If you think about a balloon, too little (air) pressure inside and it won't inflate properly, too much much air pressure inside and it pops. For sunspots there is a similar balance between the magnetic fields that are in the sunspots and their surroundings: wrong balance and then the spots either don't form or break themselves apart very fast.
The second answer is simply, ``we don't know'' why some sunspots become really huge and some stay tiny. We have some ideas about why they never get so big as to take over the entire Sun which have to do with how fast the Sun is rotating and the details of the structure of the solar convection zone (inside the Sun, near where the magnetic fields are stored and re-generated). But we are actively researching why sunspots can be big, little, simple or very complicated.
There are few if any sunspots on the sun right now because we are at the minimum part of the 11-year activity cycle. I'd suggest going and reading some of the posts on that topic for additional details. cheers, -KD