First, there is no photo; with less than 10 posts you can't include photos on posts. The way around this is to create a photo album from your profile page, make a post saying that you did this, and then someone can retrieve & post it for you. Second, crankset bearings don't fall off, you need a photo to show what you are referring to (did the balls fall out of the cages maybe?). Third, ball bearings are usually a common size. Hopefully, you still have an old one that you can measure with a hole gauge or a good scale. (FWIW, the most likely size of those is 5/16".)
John