You use varistor wich has the max voltage for that aplication example:
vacuumcleaner
max allowed voltage is 240v ac so you chose aprox. 250 V varistor.

transient varistor

How i determine correct size varistor for protect system which work with 220 v AC?

I think your answer is given in previous question -( correct size of varistor across 24v coil)

I am just pasting the same here. . .

"When the voltage exceeds the clamping voltage of the varistor, it conducts current, preventing the voltage from reaching a dangerous level. If the elevated voltage is maintained for too long a time, the varistor will heat to a point where the metals in it melt and fuse together, causing a short. This also poses the posibility of a fire risk, as the current is now conducting heavily through the varistor. This is what often happens to a varistor in a power strip when lightening strikes nearby. It renders the power strip useless. But the connected devices are usually spared. "

Varistors are not polarity sensitive.

Now I'm pretty sure that the device in question is a varistor. I googled for SAS-820KD05, found several references but unfortunately no data sheet - maybe you have more luck.

One main criterium for selecting a varistor is the varistor voltage - as long as the voltage applied to the varistor is below the varistor voltage, it will behave like a capacitor. When the voltage is above, e.g. caused by a transient, it acts as a resistor, shorting this transient. Another important value is the max. pulse current a varistor can withstand.