I saw some comments to the effect that it wouldn't hurt to add just one more word and make it only large cities and large counties but that sounds awkward to me.

Parallelism is usually a desirable thing, but it is definitely possible to go overboard.

I started googling around for an actual grammatical term such as "implied parallelism" or "accordioned parallelism" without much success, but I did come across Mr. Clarity's style blog, which points out the great example of Winston Churchill's famous speech of June 4, 1940, where he uses parallelism to great effect, but only where it's really important:

Even though large parts of Europe and many old and famous States have fallen or may fall into the grip of the Gestapo and all the odious apparatus of Nazi rule, we shall not flag or fail. We shall go on to the end, we shall fight in France, we shall fight on the seas and oceans, we shall fight with growing confidence and growing strength in the air, we shall defend our Island, whatever the cost may be, we shall fight on the beaches, we shall fight on the landing grounds, we shall fight in the fields and in the streets, we shall fight in the hills; we shall never surrender…

There is a lot of parallel construction here, but if Churchill had not preceded all of those repeated "we shall"s with the implied parallelism of "we shall not flag or fail", perhaps the impression would have been long-windedness rather than iron determination.

Perhaps. But it does show that parallel construction isn't necessary in absolutely every case. In your case, you can get away without the second "large".

I saw some comments to the effect that it wouldn't hurt to add just one more word and make it only large cities and large counties but that sounds awkward to me.

Parallelism is usually a desirable thing, but it is definitely possible to go overboard.

I started googling around for an actual grammatical term such as "implied parallelism" or "accordioned parallelism" without much success, but I did come across Mr. Clarity's style blog, which points out the great example of Winston Churchill's famous speech of June 4, 1940, where he uses parallelism to great effect, but only where it's really important:

Even though large parts of Europe and many old and famous States have fallen or may fall into the grip of the Gestapo and all the odious apparatus of Nazi rule, we shall not flag or fail. We shall go on to the end, we shall fight in France, we shall fight on the seas and oceans, we shall fight with growing confidence and growing strength in the air, we shall defend our Island, whatever the cost may be, we shall fight on the beaches, we shall fight on the landing grounds, we shall fight in the fields and in the streets, we shall fight in the hills; we shall never surrender…

There is a lot of parallel construction here, but if Churchill had not preceded all of those repeated "we shall"s with the implied parallelism of "we shall not flag or fail", perhaps Churchill would have emphasized long-windedness rather than iron determination.

Perhaps. But it does show that parallel construction isn't necessary in absolutely every case. In your case, you can get away without the second "large".