I, too, used a large artist's canvas for a terrain base - actually three of them, each starting at 6' wide by 24' long. And, believe me, such large pieces will shrink since they are too big to build a frame for. So, if you go that route, allow for the shrinkage. My shrank a few inches in width but as much as two feet in length. They were two sea mats and one land mat for a gigantic pirate game.
I did give some thought to using framed artist's canvas as terrain bases but decided on the above instead. The supply of framed canvas will be used to paint some backdrops for games - and photographing of miniatures.