Can anyone explain logic behind size of rebel stack spawning.

[Arinsar]

Right from the start Poland has 29 force limit. If you annex Mazowia, you get around 31. If you accept Lithuania under PU, you get around 2-4 more. if you ineffectively wage some wars, you get at least around 2 (because of the autonomy and ineffective conquering). SO, you have 29+2+3+2 = 36 troops at least.
36k > 20k
You have probably miscalculated something. Besides, by 1470 Your combat width is 22 max. So, those 30k don't really fight all together, and you can simply start with 22, and reinforce with 36-22 = FOURTEEN MORE TROOPS, or just ineffectively fight with your 36 VS 30 rebels.

There is one thing missing in your calculation : you have to be able to afford that standing army for 25 years...

 

[YuriiH]

There is one thing missing in your calculation

Well, no.
Hire troops, don't pay them in full, activate when you need them to crush rebels or before any war.
Don't thank for the hint!

 

[kgmi]

Best thing is when they spawn 25k+ rebels in 3 dev siberia or desert provinces, like every single man, woman, child and animal in that region just picked up arms to rebel against you?

 

[Apollyonna]

I do not understand all the answers as to how to beat them - it's besides the point. The original question was quite clear and I too would like to know the reasoning / calculations behind the size of rebel stacks (and specifically the *ç%"%ç"* dutch revolter stacks)

 

[Me_]

Maybe @Trin Tragula or another Dev could take a look at those events. There was a point in the game's history when rebel sizes were recalculated but a few events were not changed accordingly (e.g. some event for Orissa) and were left with unusually large rebel stacks. It may just be an oversight.