What is the ideal bulk size formula in ElasticSearch?

I believe there should be a formula to calculate bulk indexing size in ElasticSearch. Probably followings are the variables of such a formula.

• Number of nodes
• Number of shards/index
• Document size
• RAM
• Disk write speed
• LAN speed

I wonder If anyone know or use a mathematical formula. If not, how people decide their bulk size? By trial and error?

There is no golden rule for this. Extracted from the doc:

There is no “correct” number of actions to perform in a single bulk call. You should experiment with different settings to find the optimum size for your particular workload.