It all depends on what pattern stitch you are using. If you are using a pattern stitch that requires multiples of 4 then you would add 4,8,12 etc more stitches to make it wider. In order to estimate yarn you would have to make a gauge swatch and go from there. I think that the lion brand yarn site has a sort of estimate on how much of their yarn you need to finish projects so you could sort of use that as a guideline.