yeah this is something I would have liked to see, it would make it a lot easier to see what tiles your city can utilize having some sort of border put around the tiles.
In the meantime, the best way I have found to know quickly the radius of workable tiles for a city is by using the 6 rule. Each side of your city has 6 workable tiles on each of its sides, they are arranged 3 wide x 2 high. So you just need to imagine the following tiles projecting from each side of your city with the center tile being in line with your city tile;
xxx
xxx
which ends up looking as wmtrexler wrote above (except a bit more symmetrical in the game since the text doesnt really make the shape look right);
_XXX
XXXXX
XXCXX
XXXXX
_XXX