help with texture

im trying to make a texture for my terrain which is

z= -1600 to 1000 long
x=-400 to 400 wide
y=0 to 800 high

im making a very basic grass with a little bit of dust here and there…

ive read almost all the stuff about textures from google…all of them talk about how to load texture frm .bmp and map it but very few talk insufficiently about how to make ur own texture

so if anyone here could give me a starting point it would be greatly helpful

also does the resolution of the texture needs to be the exact same as my image?