var value = "Входная строка";
var length = 1700; // количество символов
int strLength = value.Length;
int strCount = (strLength + length - 1) / length;
var result = new string[strCount];
for (int i = 0; i < strCount; ++i)
{
result[i] = value.Substring(i * length, Math.Min(length, strLength));
strLength -= length;
}
var rez = string.Join("\n", result); // Результат