10MB is fine, swift can scale to billions of objects.

Read throughput will be faster with larger chunks, because of backend connection overhead it's naturally slower but also for dynamic large objects there's a default artificial rate_limit_segments_per_second which is turned up kinda high - like 1 req/s after the first 10 segments.

Write throughput is faster with smaller chunks. Not only can you have more workers uploading parts of the object simultaneously, you don't have to upload as much to make a single in place update update to a chunk.

Be careful with "synchronization" and "only changed parts" though; since swift prefers availability to strong consistency, read-update has some caveats.