15-17" , aiming for at or below 16". Better to have it a hair short and throw an extra armload or two on top than a complaint that it won't fit.
You can always ask your customer what kind of wood burner they have, what size it can handle versus what size they prefer, and offer a custom cut/split option for future sales or seasons, just make notes accordingly in your books.
.Our wood furnace takes 20" just right, but its easier to just cut everything one size.
I tried a tapemeasure and hatchet once, bah. Then I made a story stick for use with a spray can, and on odd length logs, instead of cutting off one of the seasoned ends, I'll flip the story stick and get one full cut from the end and put the odd/short cut elsewhere in the log, like at/near a defect. It's not as precise as a Mingo, but it works for me, and uses up all the oddball partial spray cans.