At this stage you could go to the In Model materials, select the texture and click On Edit. Then edit the dimensions. If I were adding your texture I would have started out by using File>Import to bring the image in as a texture and I would have applied it to the face starting at the lower left corner and then dragging across to the right edge. That would make it the right size from the beginning.
By the way, maybe you could update your forum profile since you aren’t using SketchUp 8.
I thought of that Dave, but that screen thing is on an angle, so it didn’t work too well. And adjusting the size in material edit resulted in what you see above.
Thank you all for your amazing help. I did manage to work it out eventually and found importing the image works, my first attempts were merely panic when I saw this tiny image appear. Then it hit me with the auto scaling. Brilliant.