If you have the right kind of pliers, and you're willing to trade off a little canvas for ease of use, you need...
the depth of the bars
plus the width of the bars
plus about an inch or two
That lets you use "gallery wrap pliers" with two forward protruding tongs that register on the inside "sides" of the bars. Piece o' cake starting with your third or fourth canvas. But if you like aggravation you can get by with just the depth of the bars plus about 2/3 the width. Don't forget the power stapler.
edit...I don't necessarily recommend those Osborne pliers, they're just an example of the type of prongs that I feel work best with gallery wrap.