You don't generally need to set both height and width as the textures are normally seamless and repeat to suit. If you have a texture that only shows one batten, then set it as 8" horizontal (2 =16", 3=24" etc. Normally you wouldn't want to stretch a texture disproportionately, so you would almost always check "Maintain Aspect Ratio" and edit one dimension to suit, to resize the whole texture.
What I was saying about pixels earlier, if you have downloaded any (seamless) texture from the internet, say it shows 4 horizontal battens and 4 boards, and you therefore want that texture to be 32" wide (4 x 8" spacing) then that particular texture will have to be 512 pixels wide (or have to be resized as a jpg to 512 pixels wide) - 32" x 16 pixels/inch
You can normally calculate how wide you want the texture to show, based on what the texture itself shows - for example if you want this texture:
Annapolis Blue.jpg 32.19KB
to have battens at 8" o.c. then it would be set at 32" wide ... and indeed, I designed it to be 32" wide ... so 512 pixels.
As a general rule, you wouldn't want to change any of the texture sizes - They SHOULD be correct as they were designed - unless they are obvioulsy wrong.
Does any of this make sense now?