How big are the pond apples?
If you mean the fruit, they vary in size. Some are small, size of a racquetball ball. Some are larger, as wide around as a tennis ball at the widest point and a little longer.
If you mean the tree, I have several, from seedlings I am grafting to up to 12 ft high trees. I used to have two even larger trees over 23 ft. high, but they were removed to make way for a stormwater feature. I wasn't complaining.
Note, I am offering these for the seeds and even the small ones have more than enough seeds for most.