Apparently GE invented the banana plug, there are 2 different sizes I think but the most common is 4 mm shaft size. If you can find a definition that would be good.
Banana plugs dimensios are total mess. Reason is that it is not at all standardized. There is only practice.
There is history also. But then we can also ask who's history. Different area in world have different trumpth of history. As also in many science there is "The Political Science of...this and that. GE launched it. If I tell Hirschmann launched it after then whos history more right. Here rumors tell that it was Hirschmann. Who have more money his history is more right or who have biggest gun his history is most right - or then not.
There is so many dimensions and not only main dimensions but also so many details dimensions. And if want more mess, after then we add tolerances to this soup. I have seen from around 1mm to around 8mm plugs in my life but highly very common is 4 and then 2mm.
But also whole big mess is also coming partially from fact that different part of world have been with differeent units. We live here with metric system but some others live with foots and fingers and inches. Then some kind of more or less accurate rounding between these.
Even if we take as called as "4mm" banana plug, there is lot of with different detailed dimensions. Who can show what are accepted tolerances - I do not mean opinions, I mean facts. Where is definition.
Even if diameter is around 4mm +/- some but then length is huge mess. But in america, I think, more common is short and in europe is long. Most common lenghts are around 15mm (around 14 - 16mm) then other very common is around 20mm (something between 19 - 20.5mm) and then there is also sopme times these 11-12mm. (and then some amount all between 10 - 25mm).
I have used these in lab mostly and these always work without any problems. There is also other models. Also Pomona do some good.
Robust and simple models are:
Hirschmann BSB 20 KHirschmann BUELA30K(And also with these, "looks like" copy products are (mostly) total crap shit - example these contact springs loose force or is originally wrong due to wrong manufacturing and wrong materials.
One copycat example about crap but cheap )
With tens of years experience in professional use I have never meet problems with
genuine Hirschmann banana plugs (or high quality compatibles). But also some experience with cheap "looks like" copy shits - sometimes learning has also become expensive.
If I recommend some banana plugs for example Siglent SPD3303 series it is just these genuine Hirschmann with "standard" ~20mm length. Kind of "industrial" references. More different more bad.
Also Siglent.Eu show these. (but are they genuine - I doubt because price)
Pity that Rosenberger, Lemo or Suhner do not make good industrial-military quality banana plugs.
Then what is idea for need big aperture in binding post. 4.5mm is well enough if banana plug is well made. I can not at all understand why they do these fat neck banana plugs. For what? Oh, perhaps audiophiles think they look more cool... who in industry ands professional work in lab or industry use these. Perhaps in some rural poor area where they do not have money for buy tools. If I type banana plug to google... omg what kind of mess and shit there can see if select show images. OMG.