Most slaves do not have a problem with the size of standard base rings. Usually they come with the size 4cm, 4.5cm, 5cm diameter. You can always find your pick.
However a few slaves just can not find the right sized ball ring. As a very experienced key holder, I will introduce you two easy ways to make the base ring smaller.
1 Buy a hot glue gun and shoot hot glue on the inner edge of the ring to directly smaller the girth. Therefore the ring becomes smaller.
2 If you do not have hot glue, you can always find thick thread/ thin rope/shoe lace available. Use it to tie knots on the ring to make it smaller. Tie enough knots until the ring becomes the right size. If you tie too many knots and ring becomes too tight, then you simply untie a few knots.