I don't believe the total volume of the pipe is a factor.
A common recommendation that I have seen says, "If your 100 feet away, than your septic inlet needs to be between 3 to 7 feet deep with five feet giving you a nice 5 percent(1/2"/foot) grade. As for distance, the rule of thumb is to put clean-outs at just under twice the distance you can reach with a snake."
There is a maximum slope which would cause the liquid to move too fast and tend to leave the solids behind. Apparently this is not thought to be it.
I have no first hand experience with this design concept. It's just all I could find in by limited research.
If someone tells you it's a certain way or not a certain way, make sure you get the reasons.
I'm sure there is more detail info available along these lines(I know I have seen some along with the research data on the capabilities of low flush toilets.). It's just time consuming to find it. I believe ASPE is a good source. That's the American Society of Plumbing Engineers.
How far is it to the existing drain line which is running from your house to the septic tank. Maybe that's where you should tie it into, if it shortens the run.
Or maybe just provide a holding tank that can be manually emptied periodically.