What size stone can I put in this tumbler?

If you have your barrel 2/3 to 3/4 full you can add larger rocks. For a beginner polishing, larger stones may be challenging. Once you have a good understanding of tumbling, you could tumble a rock the size of kiwi. Possibly a bigger rock but it is going to be challenging. If you don't have enough filler rock you can use Non Abrasive ceramic media.