Correctly determining the length of the front line can be painstaking and time consuming if you track all the twists and turns in a front line. I am not going to do that, but instead calculate the distance of the front line based on rough point-to-point measurements using the measuring stick in Google earth. Feel free to tweet me a more precise measurement.
Starting north from the Russian border down to Troitske is little less than 6 miles (10 kilometers). The Russian defensive line then runs from Troitske (pop. 7,241) for 36 miles (57 kilometers) due south to Svatove (pop. 16,420) and then for 27 miles (43 kilometers) down to Kreminna (pop. 18,417). This line is at least 69 miles (111 kilometers) in length.
From Kreminna down to Bilohorovka is another 8 or so miles (13 kilometers) and from Bilohorovka to Bakhmut is 25 miles (40 kilometers) and from Bakhmut to Avdivka is 34 miles (55 kilometers) and from Avdivka to Marinka is 17 miles (27 kilometers). The distance from Marinka to Pavlivka is around 19 miles (31 kilometers). This makes this stretch some 103 miles (166 kilometers). It is probably a little longer than that due to all the twists and turns.
The front line then turns west and runs to just north of Vasylivka, for at least 89 miles (143 kilometers).
Lastly is the section of the line protected by water obstacles. This is about 120 miles (193 kilometers) protected by the Kakhovka Reservoir and then some 60 miles (97 kilometers) of its length protected by the Dnipro River. At that point we are at the Dniprovska Gulf, and there is a spit of land on the opposite side that stretches for 36 miles (58 kilometers). We are not going to consider that spit of land to be part of the front line.
So, I have the entire front line being 69 + 103 + 89 + 120 + 60 = 441 miles (710 kilometers). The actual real line is probably longer, maybe up to 25% longer counting every twist and turn (please someone have at it). The 120-mile stretch across the Kakhovka Reservoir probably does not need to be seriously defended, maybe a few AA batteries and some mobile forces. So, in all reality the front line that needs to be seriously defended is 321 miles (517 kilometers).
Now, Ukraine actually has to defend more than that. Because Russia can choose to initiate an attack across the border into northeastern of northern Ukraine at any time from Russia and Belarussia. So, Ukraine obviously has to hold defensive forces around Kharkov, Sumy, Chernigov, Kiev, etc. The requirement is really not the same for the Russians. I do not think we will see a Ukrainian drive on Kursk.