little & large - a challenge

[andrewnummelin]

The decision to participate in the Scalefour Society's "Socially-Distanced Challenge" led to me thinking about something appropriate to the Circle.

So what were the very smallest and largest rolling stock items used on a standard gauge Welsh railway?

Plural because size could be defined in different ways (length, volume, mass, ...).
Please exclude articulated or permanently coupled sets (did the GWR's Pollens work in Wales?), and things that would not appear in a revenue earning train (so not platelayers' or Wickham trolleys).
Finally, being a Research Circle, please don't simply quote from Wikipedia or similar. References should be to original rather than secondary sources

[Rhobat Bryn]

If rolling stock includes locomotives (which I believe it does), my opening bid for largest category would be a Barry Railway Class D 0-8-0 tender engine