Calculate burden resistor value of current transformer and measure ac current with arduino
There is no "You need X resistor for Y transformer". The resistor you choose depends on what current range you are working with and what voltage range you want that to be expressed as.

That transformer is a 100:5 (or more properly a 20:1) ratio.  That means for every 20A going through it you get 1A out. 

That output current is then passed through a burden resistor and you measure the voltage across that resistor.

If you are, for example, working with sensing up to 1A current then the maximum current you'd get out of it would be 1/20A, or 50mA.  If you want that to be expressed reliably within the range of a 5V ADC then you would:


Work from the mid-range of the ADC as 0V to give ±2.5V (assuming AC)
Calculate the resistor that gives 2.5V across it when 50mA is passed through it
Pick the next-lowest common resistor value.
Reverse the calculation to get the voltage range you now have.


In this example, R=V/I, so 2.5/0.05 = 50Ω.  50Ω resistors aren't common, but 47Ω ones are, so you'd pick that.

Then reversing it gives V=IR = 0.05 * 47 = ±2.35V

Give it a 2.5V offset to the ADC and you can now read points on a sinewave between 0.15V and 4.85V at 1A sensed current.