Terrain Question

Im wondering what i should do for mapping scale on my simple terrain texture thing Screenshot by Lightshot i have a terrain resolution of like 6k by 4.2k image and for mapping scale your supposed to put it at the resolution of it what am i supposed to do if its not exactly square though? Screenshot by Lightshot

Does not need to be square.

Number of components (8/8), 10/7 will make a rectangle to match 6k / 4.2k, just scale the land to match then.

Where would I go to change this