For the lines, you could try to print it as a Vector graphic. You can select that in the printing dialog. I don't have a lot of experience eith scaling, but I would recommend that you first measure the width of your head and then measure the width of the helmet with the measure tool in pepakura. The helmet should be wider than your head to allow for padding and some tolerances. Mine is sized up a lot, but i think 3cm on each side should be enough. Before you build it, you should check heigth and depth of the helmet as well, just to be sure. You should also be able to make the helmet out of foam, although I haven't tried that. You have to remember though that foam has a substantial thickness while paper doesn't, so you have to adjust the tempelate to allow for foam. You also should try building something smaller out of the foam before to get the hang of it, if you don't have a lot of experience with foam. Also, if the you have is very detailed, you should maybe look for one with less details of you use it with foam, because you propably won't be able to create too much detail with foam. The finer details you have to carve into the foam, I think.