Please follow the below steps to change the size of NSX from Large to Compact.
1) Login on the VC with the user name and password
In my demo lab my VC IP is - 10.0.0.10
Hence the URL would be :- https://10.0.0.10/vsphere-client/?csp
2) Click on Home - Networking and Security and click on NSX edges on the left hand pane.
3) Select the edge for which you want to change the size and click on Actions on the top.
4) From the drop down action menu select the option - Change Appliance Size.
5) In the Change Appliance Size option, choose the size as Compact from the given choices.
Note:- We have 4 sizes available for NSX in 6.4 Version
c) Quad Large
d) X Large
6) During the process the NSX edge would get redeployed with the new compact size.