What's "best" is entirely subjective. What do you have to work with? That really determines some "best" options.
I just have vCenter. So is the "disk usage" on the snapshot tab represent the snapshot size?
Yes, disk usage reports the snapshot usage that contains all disk resources as part of it. So if you opted to snapshot the memory as well, that factors into the calculation.
There are some vRO workflows out there to report snapshot sizes of a VM.
For example, check the workflow attached to the following community thread - https://communities.vmware.com/message/2081161#2081161
Or this one - https://communities.vmware.com/message/2399717#2399717