Red hat also recommends a shard size of 512mb, it's actually the only shard size they support. Also check the chunk size on the LVM thin pools running the bricks, should be at least 2mb. Note that changing the shard size only applies to new VM disks after the change. Changing the chunk size requires making a new brick.

Regarding the chunk size, red hat tells me it depends on RAID or JBOD

https://access.redhat.com/documentation/en-us/red_hat_gluster_storage/3.5/html/administration_guide/Brick_Configuration

chunksize

An important parameter to be specified while creating a thin pool is the chunk size,which is the unit of allocation. For good performance, the chunk size for the thin pool and the parameters of the underlying hardware RAID storage should be chosen so that they work well together.

And regarding the shard size, you can fix that with storage live migration right? Use two volumes and domains and move them so they will adopt the new shard size...

Am I correct that when you change the sharding on a running volume, it only applies for new disks? Or does it also apply to extensions to a current disk?