How to know size of an elasticsearch index using Kibana?

Goto Kibana and then click on "Dev Tools" from Left Menu, Once the console is open run the following command to see size of the given index.

GET /_cat/indices/<index_name>?v

For example if your index name is users then you should write the following

GET /_cat/indices/users?v

Then you will see output something like below

health status index uuid                   pri rep docs.count docs.deleted store.size pri.store.size
yellow open   users fDYh8frAQ5207XR79iynxQ   5   1        102            8     55.5kb         55.5kb

FYI