crank bearings broken?

Do you live near a Local bike shop?

Do you have a caliper measuring tool?

When you say 'they are the same size', how exact are you being?

A couple of things could cause you to choose not to - being mismatched size - being worn down...

If it were me, I'd replace the whole lot of bearings in one go. If they haven't been damaged by the disintegration of the one, they at least could be worn/damaged or fatigued and ready to fail...

I'd clean that sucker out really well, regrease and repack it with fresh bearings. if you can't get bearings at your local bike shop, try an auto parts store or ask a local friendly mechanic (if there are any).

For short-term use, you could consider repacking the bearing with clean new grease and leaving the one ball out. I usually don't run less bearings if they are radially load-bearing, but if they are all bearing the load (ie headset bearings), running one less bearing isn't usually the end of the world.

If you race, replace... it's not very expensive to replace compared to the value of losing a race due to an unexpected failure of something critical like the BB - particuarly as it's not exactly a road-side friendly repair.

First inspect the ball race if a canal was formed by the broken bearing. If not too deep then change all ball bearings but make sure that they fit snugly with enough grease. Why not change into a sealed type bearing? It is stronger and cheaper in the long run 'cause it requires less maintenance and it doesn't get stuck-up.