As a general rule of thumb, the length of the string post needs to be at least 3/16" (5mm) longer than the thickness of your headstock. Otherwise the string post hole won't fully clear the top of your headstock and inserting strings will be difficult.
Non staggered Tuner Upgrade Kits (both the 3+3 and 6 Inline) use .827" (21mm) string posts by default. This works with the majority of guitars. The Staggered Post kits use two .750" (19mm), two .787' (20mm) and 2 .827" (21mm) tuning machines.