It's up to you to choose the size of each twist section; just make sure they're as uniform as possible. While a rat-tail comb is useful for creating nice, even parts, you may find using your fingers to make parts easier once you get used to this method. These parts don't need to be exact, but if you want to make them very neat around the front of your head, pull the comb out then.
I finger comb each individual twist section before twisting, while also using a holding agent. Here, I'm working Organic Root Stimulator Lock & Twist Gel through each section to provide a light, non-sticky hold.