I need some help with 3d textures from file.

This is a question for the more advanced users, as I am more advanced, yet I don't have time to look for the section for advanced users.

I need to create a 3d texture on a plane, using a picture.

What I am having trouble with is the scaling of the image I bring in from file.

I need it to fit to the plane its a polygon plane, and it must be, as I need to combine the polygons afterwards.

So what I am asking is, how do I get the image to fit to the plane and not to be 600X the size of it?