You can easily measure it yourself, right? You should know which component you are going to use, and you can read the datasheet and compare it with the footprint dimensions.
EDIT: there may be some standard in naming those footprints, but I don’t know what it could be.
I have been reading datasheets and measuring parts for longer than most of those here have been alive. I started making PCBs with paste-on donut holes and black tape on a Mylar grid. The only thing that is new to me here is KiCAD.
The only thing I was asking about was how the NAMES in the footprint library were created. The naming convention! That’s all.
I just asked a simple question that was stated in very precise English. Has anyone actually read it instead of ASSUMING that I was asking something else?
Forget the question. I will figure it out for myself. Thanks for …
Well, I don’t know what for. But thanks anyway.
@ChipsAndChips : Already the third answer (and again the 9.answer) stated that 4x3 for your example smd electrolytic cap refers to 4mm diameter x 3mm height. And it was written in very precise English. Do you actually read it?
Forget the question. I will figure it out for myself.
This solution is often faster than writing the first thread-opening text.
For the patience of those who answered your question time and again. Your tone and attitude is inadequate and not conducive to anyone helping in the future.
Hi @ChipsAndChips ,
As I stated above, the dimensions were in millimetres and were height and diameter. To confirm which was which, use one of the measuring systems I mentioned above to measure the diameter, which is the grey circle in the footprint which I also mentioned above. However, as I also mentioned above, different brands of electrolytics have different sizes, so, unless you know the brand of electrolytic you are purchasing and have the data sheet for that electrolytic, it is wise to first obtain then measure the electrolytic before finding a suitable footprint from the Kicad library.
What I did NOT mention was, YES, you do need to go into the footprint editor to confirm the measurements of the footprint. All my comments were regarding the footprint editor as can be seen in the screen shots I posted.
You wrote about footprints, so I assumed (incorrectly), you had ventured from the symbol editor to the footprint editor.
My apologies.
Finally,
ABSOLUTELY!!! in my case.
Edit: I also use my own symbols. 
KiCad has the KiCad Library Convention, and it also has some suggestions for filenames.
I never select a footprint in that dialog, i always just place the symbol with no or the default footprint and then (if necessary) use the assign footprints dialog later, which I believe shows the footprint descriptions.
Hi @ChipsAndChips
I feel your pain ! I do not draw my own footprints for the jellybean passives either 
and a good few years ago I had the same head scratching moment as you are enduring. I have no idea about naming conventions but I realized that it was diameter x height and the units are ‘mm’ also the information for the dimensions can be found in a multitude of places which is great as you will find it at any point in your workflow. I use a lot of the capacitors that you are choosing in various sizes, lets say a 6.3x5.4 I roughly measure the product in my hand and read 6x5 I know its the right footprint. A final point with the SMD caps and res there is an option for hand soldering that provides bigger footprints. I hope this helps a little 
Andy 
jmk, My apology. I apparently missed your short answer, “mm”.
I was too focused in trying to find the screen you posted a photo of. I still haven’t found it. Perhaps there have been some revisions in the recent versions. I am presently using V 6.0.07. I need to download the later version, perhaps tonight.
The screen posted is the footprint editor.
The item will show but you will need to extend the RH border to read the description.
I have apologized to jmk. I missed his short, but accurate answer in a longer post. If I offended anyone else, then I offer my apologies to them as well.
I am re-reading the thread and, so far, I do not see that answer anywhere else before your post which I have quoted. I will continue re-reading.
Comprehension of written instructions on specialist internet forums can be difficult.
Assumptions are made, pronouns can confuse, words missed or incorrectly interpreted, different levels of experience, English Second Language translations etc.etc. The list is almost endless. 
Usually everyone finally makes it to the finish line.
Have you found the “Footprint Editor”?
The second number is the height? What does the height have to do with a footprint? Are the footprints actually 3D models?
I am sorry, but my mind tended to regard any reference to height as being bogus. Perhaps someone can explain that. And if so, can this be seen in the Footprint Editor?
The second number is the height?
yes.
What does the height have to do with a footprint? Are the footprints actually 3D models?
Every footprint has a 3D-model assigned (or at least can have a footprint assigned - some 3D-models are not delivered in the standard lbraries). So if you want a correct 3D-representation from your board the height could be important.
And if so, can this be seen in the Footprint Editor?
in the footprint editor you have 2 options:
With option 2) you have also the ability to assign an own 3D-model, or scale/move/rotate the 3D-model .
To be honest I also mostly ignore the height-value of footprints (and my personal ibraries don’T differentiate between different-height models), but I accept that it’s valuable for others.
So I am reading the library rules and guidelines and it seems that they were violated in the library I was using.
"G1.9 Dimensional Units
When specifying a dimension (e.g. in a footprint name) the units must always be provided:
3mm - millimeters
1in - inches"
Apparently someone did think about this but the author of the library did not get the memo.
Just an observation.
KiCad has the KiCad Library Convention, and it also has some suggestions for filenames.
Library Conventions | KiCad EDA
Library maintainer rules & guidelines
Thanks, good to know.
That option for a bigger, hand solder-able footprint sounds useful. How do I find it? Is it a different library?
Wow! Who would have guessed? Thanks!
Not all footprints are drawn with additional hand-solder-version. (for instance the polarized smd-caps are only delivered with one version).
In the standard-libraries the handsolder-version is placed in the same library as the normal version. Look into the already used Capacitor_SMD library: for simple 1206-ceramic caps you find:
No need to guess… Explore the site.
Explore the site.
Dimensions are shown… Explore the site.
The library is reflecting manufacturers Data Sheets. Height (length) and Diameter are always quoted in those data sheets. Here is just one of dozens of examples.
This thread is getting ridiculous. Learn to use the site and read the documentation and familiarize yourself with the program.
I think it is time to close it.