Try this:
Google 'Standard Pipe Nipple Lengths"
Anvil has a graphic showing a chart for (threaded) Pipe Nipples from 1/8" NPS to 6" NPS with standard lengths from 3/4" min. length for the 1/8" NPS size and 3-1/2" minimum length for 6" NPS size'
Other standard sizes range from 1-1/2" to 12" including the following incremental lengths: 2", 2-1/2", 3", 3-1/2", 4", 4-1/2", 5", 5-1/2", 6", 7", 8", 9", 10", 11" & 12"
They also state "Other sizes available"
This is all very logical because: a Nipple is just a piece of Pipe cut into short lengths.
Sometimes its possible to do all the right things and still get bad results