Your goal is to enhance the length of your nipples. You want to gain permanent length of the nipple itself and don’t pump the areola. As you pointed out correctly. The diameter is critical. This video below explains my nipple method. I am using those Thickwall cylinders. The smallest and the next bigger ones. 1/2 inch (1.4cm) “Bullet Nips” and 3/4 inch (2.0cm). I personally think that 1 inch (2.8cm) as nearly too big unless you want to pump the areola. But you always will get a red ring around the areola pumping with such big diameters.
I hardly fit in the smallest Thickwall cylinders. Only the tip of the nipple gets sucked in. So you should use silicone lube to make the nipple move inwards easily. Because this is what will happen. Over 10-20 minutes the nipple will be stretched significantly and much more of the nipple will be inside this 1/2 inch cylinders. This way you definitely an lengthen nipples. I like to switch diameters because also a thick areola can be fun. But you should do both.