How large could king Arthur's kingdom have realistically been?

Question

I fully understand that most what is written about king Arthur is shrouded in myth, many stories having been written several centuries after his time period. Most material I found focuses on the stores themselves, or mentions the geopolitical landscape but without containing enough information to build a realistic image of a domain a 6th century king or warlord or whoever the Arthurian legends were originally based on, could have had in that area. What I'm interested in, is how much area he could probably have controlled, the number of towns, villages, castles, and a rough estimate of the population. If written material is scarce from that time period, and what we have usually doesn't focus on these kinds of information, what can we infer from what we know about typical kingdoms, lifestyle, economy, etc. of that time period?

Answer 1

As you notably stated in your own question, King Arthur's Kingdom of Britannia was MYTHICAL. It was fake. Once that is understood, we can move forward.

There are numerous different beliefs about how the myth of King Arthur developed, and who it is based on (with my personal favorite being that Romulus Augustulus of Rome went to Britannia after being deposed).

Considering that many of the names of characters in Arthurian legend are, to varying degrees, Celtic and Welsh, I think it would be fair to assume it is of a similar size, albeit smaller than the mid late 700s petty kingdoms and larger duchies. Similarly, the earliest records of King Arthur are Welsh and Cornish, so I feel that, if I ever get the opportunity to rewrite the Matter of Britain, I would make him king of Gwynedd, in North-west Wales.