guitarpick said:

i've always been curious to know how the sizing of nails in hardware
stores came about. hopefully you tool experts can help me out. who
came up with the system, where did it come about, when, etc. thanks.

The system of 16d, 8d etc was based on the cost of the nails.
16 penny, 8 penney was later replaced with d.
Exactly when, I'm not sure, but I think it was sometime in the late
1800s in the USA.

FWIW,

Greg G.