I would like to know how the performance of the ElasticSearch service decreases/degrades with in size of the data increasing. Are there any metrics which indicates this?
It depends on
I would suggest you to start with ElasticHQ and list of diagnostic metrics
and Marvel from Elasticsearch
It will give you an idea of potential bottlenecks
Fill free to ask more specific questions when you have them
ElasticSearch Usage Statistics from elasticsearch hq plugin