There is actually a ''proper'' canvas for canvasing a deck. There used to be a company who advertised in WoodenBoat Magazine that sold both white lead and canvas for canvassing as deck. I beleive that Chappelle''s ''Boat Building'' has a good section on how to canvas a deck but with my copy in storage I can''t say for sure.
When I wanted to do the deck on my Folkboat, I could only find 54" wide rolls of canvas and so needed to seam the canvas on the centerline. Length was no problem.