It depends. You ideally don't want it so small you end up having to make a crazy number of requests for larger uploads. For a 1GB file, you'd need to make 100 requests to finish uploading a file, which is a decent bit, but as this isn't sustained traffic it's not the worst thing. People with faster connections may hit you with a large spike of requests when uploading larger files, as a gig line can push 10MB of data in no time at all, so just need to make sure you can handle that if you want to keep it at a lower value.
You could increase it a bit if you wanted to reduce that overhead a bit, as 10MB is pretty low. But unless you're noticing performance issues, particularly when uploading files, it's probably not a big concern.