Well as far as sample size all the polls I see on RCP/538 have sample sizes of like 900-1600 likely/registered voters so I dont see how you couldnt pull together the same sample size for a county and have it be more accurate (for that county's results) than for the a proportionately smaller sample of a larger body.
I figured like a local news organization or school would want to do some for major counties (cuyahoga and like 2 other counties next to it have 10% of the votes in the state and I was trying to see if the partisan strength was the same there or not) in the state. There are a lot of things that could influence the outcomes in a state and rather than just take RCP avg + 538 avg and click a state into a column I was trying to look at last presidential election result [candidate and strength], current senate polls, whether bernie or hillary won the primary etc. In ohio I saw that a handful of counties helped put Obama at +3% but if Hillary doesnt perform as well as Obama with minority voters or just doesnt motivate democrats like Obama did in those counties the state could go differently. I guess I just wanted to wonk too much
Thanks for the link to the tampa bay polls.