Because 10Gb is required in a Nutanix cluster it will always default to 10Gb, it can be customized but I'm not sure why you would.
@vivekds 10GB if you only use 10GB switch connected. 1 GB if you only use 1 GB switch connected. If using both you can specified which VM to use 10GB or 1GB. You can see openvswitch assignment from Network tab on Prism