Hipshot Tuners in 2017 SE Custom 24

[HardRockX]

I would like to purchase Hipshot locking tuning machines for my 2017 SE Custom 24. I can get a PRS universal mounting plate with them, but I'm unsure what size string post length to get. They offer 19mm, 20mm, 21mm and 23mm. Does anyone know what size will work best? Thanks.