I read somewhere that the radius is equal to the length of the tangent . But how and which tangent ?
It wasn't explained there.
I think it is incorrect. Please provide some insight into this.
Yes, it's equal to length of tangent. It's any tangent, length of tangent drawn from radical center to any circle is the same, that's the most important property of it. Radical circle is basically point of intersection of two radical axes.