The rules state an area of 48 (8 x 6) 150x150mm squares - 1200x900mm for the 15mm game.
I've decided (rightly or wrongly) to reduce the square size to 100x100mm as my figures are on much smaller bases (25x20mm rather than 30x30mm).
This would mean I need an area 800x600mm. I found the answer in Hobbycraft - an 800x600mm ready prepared canvas.
I started by giving the canvas a coat of cheap acrylic green paint.
Next I gave it an all over wash of diluted antique oak coloured satin varnish.
Then finally I marked out the grid with a Sharpie permanent pen.