I'm the training manager for a $150M+ company (400+employees) who is ISO Registered, a state level Malcolm Baldrige Award Recipient, and we're doubling in business every three years.

We still have no dedicated training department and I'm looking for benchmarks in regards to what is the average company size (workforce/sales) before justifying a sole department?

You may like to have a look at this conversation on trainers to staff ratios which you can then use to calculate if you need to form a department based on the number of trainers you need. Department size is usually relative to company size and also the size of the staff physically accessible to the department which you need to take into consideration as well.

Hoe this helps.