The 105mm length works because it's just long enough to brace against the hand as needed when held between the finger tips. Mine is a long-fingered man's hand and the olpr pen is long enough to reach. I have found the tiny 5.5mm diameter barrel is acceptable for grip. Skinny but not annoying. As someone who prefers to use a fountain pen but needs this tiny ballpoint as a simpler more reliable alternative, the writing feel and experience handling the pen is important. This tiny olpr pen seems fully adequate for basic writing so far.
It balances the need for a tiny enough pen to carry with you in your Pocket journal or Mini XS journal, and have it there with you all the time without making your journal bulky. Thumb-click operation makes it fully useable when you need to jot a note standing up when you're not at a desk--such as in the grocery produce aisle jotting a number code. A thumb-click is nothing revolutionary but it's a thoughtful inclusion when an ordinary Bic or Biro requires two hands to un-cap. With the olpr pen you don't need to set down your journal to use the pen so you can write a note any time.
Some will wonder how this olpr pen compares to the Zebra Mini T-3 olpr has listed as an option with their wallet/journals. Many of the parts seem interchangeable so the olpr may be a development of the T-3. One significant improvement the olpr pen makes is that the olpr click is smooth and secure while the T-3 has a wobbly click and a wobbly clip. The T-3 clip bends to the side every time I press it to close the pen. Olpr seems to have fixed this problem entirely because the olpr pen clip is sturdier, straight, and doesn't displace to the side when you click it closed. The two concentric barrel pieces of the olpr version feel much better fitted and operate smoothly and stay straight. No wobble.
The olpr pen appears to use a fine point, or at least it feels similar to the T-3's 0.7 fine point tip. Both pens appear to use the same size/type refill, so medium T-3 refills likely fit the olpr pen if you want to try a different line.