I do have a research document that I made months ago, but it doesn't really make sense to share these numbers since I've modified them while working on the mod (also there's like 40 different sheets for all the countries and resources and I didn't really format them to be shared).

Most numbers are based on production statistics from the time period, mostly from the early 20th century since that's when international statistics were actually available.
Those statistics are mostly sourced from various editions of the Statistisches Jahrbuch für das Deutsche Reich, but there are other sources, too (they were collected by an organization based in Rome).

Of course I used many other sources as well, both from the period and modern descriptions of historical production. My favorite source out of all the ones I've read is this informative article from 1919 about iron production in Lorraine and worldwide.[www.jstor.org]

When turning real-life numbers into in-game numbers, I initially based them on real-life worker numbers (e.g. Germany had 9.6 million people working in agriculture in 1925, so I would expect Germany in those borders in-game to have enough levels to be able to employ that many people), but then shifted more towards basing them on typical vanilla in-game values once the leak came out.

My values ended up cutting the available resources in half compared to vanilla numbers on release, I'm not sure if they greatly increased their values since the leak (and I'm too lazy to look it up) or if I just got the overall numbers a bit too low.
Either way, I think my values work fine and vanilla is pretty crazy at times with how many levels of mines they hand out to random states.

As for agricultural resources, I noticed after playing the game that food demand really doesn't scale as much as food production scales with tech, so I set the number of agricultural resources pretty low compared to where I had arable land at before.
I'm pretty sure that Kiev, the state with the most potential for grain farms, can still feed all of Russia on its own when maxed out and at max fertilizer tech, even with my lowered numbers.
So giving any state like 500 grain farms/livestock ranches, which is where my previous numbers were at, would be complete overkill.

Yeah, I agree. Paradox did overkill some resource numbers.

European Historical Statistics, 1750-1970
edited by B. R. Mitchell
is something that might interest you