Having owned a 24, 19.5, and 18 inch Grendel, I will stay with the 20 inch barrels (19.5-21 inch). The minor velocity loss isn't too much of an issue when using the right bullet, and powder. If I can keep a 123 scenar above the transonic range out to around 1200-1300 yards (give or take 200 depending on atmospheric/environmental conditions) I am happy with that. I rarely shoot much past 800 yards anyhow, and when I do, I usually use a bolt gun. The 24 inch barrel was simply too cumbersome, clumsy, and awkward to be reasonably handy. I primarily use the Grendel for practical rifle matches shooting steel out to about 550 yards. The 24 inch barrel takes too long to insert into, and withdraw from ports, it doesn't swing as quickly, and once started, it is harder to stop it from swinging.
If memory serves me right, there is a loss of 10-25 fps per inch of barrel removed. If my handload already generates 2480-2550 fps (depending on temperature) with a 123 scenar, that gives me plenty of reach for what I need. If I were primarily going to be shooting prarie dogs from a bench rest, then the 24 inch barrel would be the way to go..but that isn't what I use the rifle for.
I would recommend considering what you intend to use the rifle for, and select your barrel length with that in mind. Bill Alexander (owner of the original name 6.5 Grendel) felt that the 20 inch barrels provided the best all around performance and handling. He has probably already forgotten more than I will ever learn about the Grendel, and similar cartridges, so his opinion carries a great deal of weight.
If I were building a rifle, I would go with a fluted, light weight 20 inch barrel, tube forend with ventilation in it. I just don't like tubes that retain heat. I would install an adjustable gas block, compensator, and consider whether a heavy buffer, captive buffer spring, or lightweight bolt carrier would work in combination with your components. The Grendel cartridge isn't like the 5.56, and some of the buffer/spring combinations for the adjustable stocks that work with the 5.56 cartridge, aren't a good idea with the Grendel.
I played around with various bullets before settling on bullets in the 120/123 grain weight for the same reason I am staying with the 20 inch barrel. If sure wouldn't go much shorter than 18 inches unless you intend to add a suppressor onto the end of it for other purposes.