If you haven't uploaded much, go ahead and change the chunk size to 20MB. You'll want the larger chunk size both for throughput and capacity. Go with these settings for Plex:
20MB chunk size
50+ GB Expandable cache
10 download threads
5 upload threads, turn off background i/o
upload threshold 1MB or 5 minutes
minimum download size 20MB
20MB Prefetch trigger
175MB Prefetch forward
10 second Prefetch time window