My breast rubbers are India rubber rings. India rubber is particularly elastic. The elongation at break for synthetic hose rings is over 300%. On the picture you can see the dimensions of my breast rings. The size is of course dependent on the breast size (your wife/girlfriend). Simply measure with a thread or tape measure and then subtract a good 20-40%, depending on how tight you want them to be. PS: Information without guarantee. You can buy something like this on Amazon, just click on the picture.