What should be ideal size of a finance function? what are the different measures we can employee to assess its efficiency

Measures to assess efficiency
If the question relates to process efficiency, an approach that can be adopted is to first identify the key end to end finance processes (Quote to Cash, Purchase to Pay, Record to Report, Plan to Perform, Acquire to Retire etc.) and subsequently identify key metrics to be monitored for each process.

The metrics can then be monitored over time to assess the level of improvement. However, to assess your performance compared to your industry, you will need benchmark data.

An organization that provides benchmark data and common metrics at the process level is APQC. They also provide some useful documentation on how to structure processes. Some content is free and some is paid. Some consultancies also provide benchmark data.

I think it's very difficult to answer the question as there are so many potential variables. However, I'd consider a few methods:

1) If possible, get some competitive information within your industry and company size. I have seen published reporting but reaching out to a finance peer community (like this) can help.
2) Track your finance spending as a % of total spending, revenue, volume or any other company relevant metric. Look to improve over time.
3) Break down into smaller sub groups. Efficiency in AP might look different than FP&A.