But come on, do you really think a 55 year old paper is going to be at the top of impact rankings when computed against current research in a field moving this fast? And, even if so, isn't it more likely this work has been superseded by others? IT'S BEEN 55 GOD DAMN YEARS, FOR CHRISSAKE!!! I think your hero worship is showing. At least find a more modern reference.
To be fair, this is a perfectly acceptable reference in the given context, and the age only helps the argument not hinders it as you suggest.
Even at 55 years old, the Feynman paper is based on known technology and physics at the time. This provides a high-end boundary to the answer that is only potentially (in this case definately) inaccurate on exactly how much lower the size can actually get.
Our tech has changed, but physics not quite as much.
What we know today about building at the atomic scale is only slightly more detailed than the rough idea that was known all the way back then.
About the only thing smaller we know of today that we didn't know back then was the details of the sub-atomic world - which I should add we still know very little about over all, and certainly not enough to build useful machines using. At a technological level nothing has changed as the sub-atomic is still out of our reach as much now as it was then.
So the atomic scale is what we are discussing.
55 years ago our photolithography methods had a 20 micron feature limit.
14 years ago our newest photolithography methods have a 0.005 micron (aka 100 nm) feature limit. That is a 4000 fold decrease in size.
Today we have 32 nm and 28 nm photolithography methods, making things about 12000 times smaller than was possible using technology from 55 years ago.
Anyways, there are more recent references out there.
One good recent paper is "Molecular Construction Limits" by Robert Bradbury, if you can find it anymore. Sadly Bradbury passed away a couple years ago and his personally hosted archive of papers fell offline. Most archived ones seem pay-walled :/
Probably the best paper on this subject is "Ultimate physical limits to computation" by Seth Lloyd at MIT.
The paper is from 2000 but his current work is on the worlds largest-qbit quantum computer also at MIT - so he is already making my sub-atomic remarks out of date.
His conclusion is purely based on physics alone and ignoring any/all technological capability.
The 'ultimate laptop' is a computer with a mass of one kilogram and a volume of one liter, operating at the fundamental limits of speed and memory capacity fixed by physics.
The ultimate laptop performs [ 5.4258 x 10^50 ] logical operations per second on 10^31 bits.
Although its computational machinery is in fact in a highly specified physical state with zero entropy, while it performs a computation that uses all its resources of energy and memory space it appears to an outside observer to be in a thermal state at 10^9 degrees Kelvin.