Check your values - I just tried in in my code:
int width = 6614;
int height = 9685;
Bitmap resImage = new Bitmap(width, height);
resImage.SetResolution(600, 600);
And I don't get an exception.
So, most likely, the values you are passing in for width and height are not what you think - have a look in the debugger and check exactly what pixel values you are passing to the bitmap constructor.
Then single step your code - as I say, it works here.