Most Yoni Eggs come in 3 sizes. Large is typically 35x50mm, medium is 30x43mm, and small is 25x35mm. The larger the egg, the easier it is for the muscles to grip. Most start with the medium size egg. 

If the egg is hard to keep in for long periods of time, do not get discouraged. It is normal for beginners. If you are still struggling with the size chosen, purchase a larger size. Once you are comfortable with that size, you can move to a smaller size.