Vacumatic Nib Sizes - The main question that people ask about Vacumatics is about the size of their pen. Oversize pens are much more desirable that the smaller sizes so it is nice to know how to tell. Vacumatics have four sizes of nibs and Oversize pens always use the biggest. It is hard to tell when you are looking at a pen and do not have one to compare with. The key is the diameter of the feed. On the big pens it is 6.7mm wide. The rest do not really matter at this point because the values are very close.