Well, let's start with upsizing. To add CPU and RAM resources is very easy. VCSA 6.5 supports CPU Hot Add and Memory Hot Plug, therefore you do not need to even shut down VCSA to increase CPU and RAM resources.
|CPU Hot Add and RAM Hot Plug|
So upsize is easily doable. But what about downsize? VCSA 6.5 supports CPU Hot Remove but RAM cannot be downsized, therefore for RAM downsizing you have to shut down VCSA decrease memory resources and power on VM. Not a big deal, just small downtime so it can be done during a maintenance window. But the storage downsize is not possible. Well, in theory, it is possible but it is definitely not supported to decrease the size of disk partitions and the virtual disk itself because it is hard and also very risky as you do not know where data are located within the disk.
Fortunately enough, we have another downsizing option for VCSA 6.5. You are most probably aware that VCSA 6.5 has introduced application-based backup where vCenter inventory, identity, and even the database is backed up to a remote location via following protocols HTTP, HTTPS, SCP, FTP, FTPS. The restore of VCSA 6.5 backup is done as a new VCSA deployment executed in a restore mode where a previously created backup is used as a restore point. The nice thing is, that during VCSA restore you can choose different VCSA form factor with different storage footprint. So this is a way how to downsize vCenter Server Appliance 6.5 storage in a supported way. The only assumption is that your backup data will fit into new storage size.
If you will plan such downsizing exercise, please do not forget to keep your original vCenter Server Appliance somewhere to have a simple way how to roll back.
Hope this is helpful and informative.