You need to understand what is thermal paste is used for and then you would understand what for the most of the time pea size is okay.
It's just a compound what helps with a heat transfer. Both CPU IHS and the radiator are not ideal surfaces, so if you install one on one without TP then there would be bubbles of air trapped in imperfections of the surfaces. They are not a problem per se but they do make the heat transfer worse, so IHS is hotter than it could be and a bit less effective to transfer the heat to the radiator.
You have two options to improve the heat transfer:
a) polish both the IHS and the radiator to a perfect surface, ie they would should mirror like; and then you should use a torque screwdriver to ensure what the cooler is tightened and leveled exactly right;
b) or you can just slap some thermal paste between them and call it done.
And in the second case you need 'just enough' of TP to smooth out the imperfections, but if you slpat too much then again you are making the heat transfer between the IHS and the radiator worse.
So if you ever find yourself minding about how much TP should you use then just start with a pea size, place the radiator (you can even screw it if that's your jam) then remove it and look at how the TP spread around the IHS. If it covers 90% with a thin film (and your radiator isn't the lowest shit tier polished with the old rusted raspel) when you are fine enough.
The only difference between 20yo and now is what IHS are way more larger so now you may find a need to use... a larger pea as a reference.
It's just a compound what helps with a heat transfer. Both CPU IHS and the radiator are not ideal surfaces, so if you install one on one without TP then there would be bubbles of air trapped in imperfections of the surfaces. They are not a problem per se but they do make the heat transfer worse, so IHS is hotter than it could be and a bit less effective to transfer the heat to the radiator.
You have two options to improve the heat transfer:
a) polish both the IHS and the radiator to a perfect surface, ie they would should mirror like; and then you should use a torque screwdriver to ensure what the cooler is tightened and leveled exactly right;
b) or you can just slap some thermal paste between them and call it done.
And in the second case you need 'just enough' of TP to smooth out the imperfections, but if you slpat too much then again you are making the heat transfer between the IHS and the radiator worse.
So if you ever find yourself minding about how much TP should you use then just start with a pea size, place the radiator (you can even screw it if that's your jam) then remove it and look at how the TP spread around the IHS. If it covers 90% with a thin film (and your radiator isn't the lowest shit tier polished with the old rusted raspel) when you are fine enough.
The only difference between 20yo and now is what IHS are way more larger so now you may find a need to use... a larger pea as a reference.