The one that came with mine was a 0.7mm tip and it was a bit too thick for me; I tend to write small on the grid lines and the ink sometimes blobbed or didn’t always dry quickly enough for me.
So I looked around on Amazon and got this guy. It’s a 4-color 0.5mm and I love it. Less smearing and I can write as small as I want.
Pilot Frixion Ball Knock 4-Color Gel Ink Multi-Pen, White (LKFB-80EF-W) https://smile.amazon.com/dp/B00IPD3KEM/
This is what I use! AND it's erasable! Pilot Frixion 4-Color... https://www.amazon.com/dp/B00IPD3KEM
Love that you got the stamp. It's the best!