Here's how we did it (sorry the pictures are a bit pale):
Plan First I drew Goliath on A4 paper and added a grid of squares on the computer - each square to be 6 inches when scaled up. (I also added some faint lines half-way across the squares to help when copying.)
Grid I drew a grid of 6-inch squares on the wall in pink chalk.
I needed a tall step-ladder and even then I was worried I'd not be able to reach - 10 1/2 foot (top of spear) is higher than you think! And I'm not tall. But I managed to reach - just.
We situated him at the bottom of the stairs as it's the only suitable place with a high enough ceiling.
Pencil I copied each square from the paper to the wall. This picture was as high as I could reach without the ladder.
The pencils finished. It was hard to get the lines smooth when drawing this big.
Inking I used black acrylic paint and a flat brush about 8mm wide. I think I used another pointy brush for the finer lines (details on armour etc). Interestingly, it was easier to get smooth lines with the paint than the pencil - for which I was glad.
And then I had to wash the chalk off the wall - I used a furry dusting thing with a long handle I found in the cleaning cupboard.
From chalk lines to this point took 2 hours, and my work was done.
Colour Finally, the talented Jenny Hamill did the colours (acrylic also) - and here's the finished result.
P.S. You can get the template free if you buy the David & Goliath PowerPoint from Lamp Bible Pictures - www.lampbiblepictures.co.uk/product/david-goliath/