Hi Christiaan
It's Tamsin, not Tasmin, but I'll forgive you.
OK - if you are using the Lines shader...
Set the texture size to 100% and then edit the shader.
You will see in the little preview window that one repetition of the tile has two horizontal repeats. SO, that's what 100% will give you - making each individual "tile" 50mm.
So, instead, if you increase the V % to 1200%, you are saying that one repetition of that preview is equal to 1200mm. That will make each individual panel 600mm as desired.
HOWEVER, I am finding that the grout width does not respond on that particular shader - making a blurry gap between the two pieces that . I believe this is a bug and will look into it further...
I had more luck using the Planks tile shader - I could get the grout joint to be really small and not look blurry. However, you will need to make the U percentage really really large to make the panels stretch along the horizontal axis.
Hope that helps!