Width of the foil relates to:
Glass thicknessThin glass requires thinner foil to maintain a neat solder line of about 3mm. Thicker glass requires thicker foil to maintain the same width of solder line. So 2mm glass would require 4mm foil, and 3mm glass would need 5mm foil.
Glass textureHeavily textured glass usually needs thicker foil, as the thickest part of the glass is greater than the thickness of average glass. This requires a little experimentation to get the one you like best, but usually is one step up in thickness from your usual. Remember you are looking at only one half of the resulting solder line thickness on each piece of glass.
Desired width of solder lineThe desired width of the solder line will also affect the choice of foil. If you like a thin line of solder, you should choose foil that is only 1 or 2mm wider than the thickness of the glass. This will give a solder line of 1 or 2mm wide. Be careful when choosing a thinner width. You still need enough foil on your glass to maintain the strength of the solder bead. If you like wider lines, a foil that is 3 or 4mm wider than the glass thickness will give a 3 to 4mm wide solder line.
Of course is possible to trim the copper foil to be thinner after foiling. This can be to even up the line, or to thin it. Gentle pressure with a sharp craft knife will cut the foil to the width you want. You may want to do this when all the pieces are assembled before soldering. This gives you the opportunity to see how the width of the resulting solder line will be. It is also the time when you can see what the line of the solder bead will be and make adjustments before begining the the soldering.