From my experience in the HDD industry I remember that a magnetic bit of data is about 13-15nm long and about 40-60nm wide (narrower tracks for SMR.) The length of a bit is constrained by the grain size of the magnetic media. However, the width of a bit (prior to SMR) is actually constrained by the size of the write head. I don't remember why, but I think it has to do with the fact that the write current is like 40 mA, and the magnetic flux density on the write element is like 1.5T (no that's not a typo)

I'm not an expert on transistor pitches, but here's a chart from Wikipedia for the 10nm - https://en.wikipedia.org/wiki/10_nanometer It's kind of impressive for HDDs considering that it's a 2 inch long mechanical arm that is able to move with that level of precision.