Unless you are using really small potatoes or making a "smashed" potato salad, you will usually slice the potatoes before boiling. If you are using a larger potato, like a russet, then you should cut them into 2 inch slices before boiling (this could mean up to 8 slices to get the pieces small enough. When slicing, do so before boiling. This is because the potato will start to break down when boiling and it will be a mess to try and cut it after it is cooked.