Here is what I have. In my case the collider is a child bp with a sphere collision (it works as well as a sphere mesh) and I have 4 of these, each one with its own spring arm. On the screen you see the one for up front.
In fact the collider is not the one which collides with the world, The probe (invisible sphere) collides with the world and pushes (or rather shrink) the spring arm. Then the collider (blue on my screenshot), attached to the arm, will collide with capsule (that is what stops the movement). You can see that the collider itself is not at the end of the spring arm.
For numbers it is hard to tell. What you can do, since it is possible to parent several objects on a spring arm, is to have a simple sphere (no collision) that has same diameter as the probe attached to the arm ( at 0, 0 ,0 location so it will symbolyze the probe, however I’m not sure if the probe size is related to radius or diameter), shrink a bit your actual sphere and place it beetween your capsule and the probe (still attached to the arm). Turn the character capsule visible and check ingame.
You will see the probe colliding with wall, pushing the collider to the capsule and you will have a better idea of sizes you need