Bit of a long shot but this is what I did at school once.
If you can get hold of a overhead projector and then shine the light on a blank wall. Blu tack the A2 paper where the light is shining on that said wall.
Then using an A4-sized clear acyrlic sheet, tape it to the top of your monitor, find the image of your choice on google, then position the image over the transparent paper and carefully trace the outline using a thin black pen (the sort that can be wiped off).
Place the traced-out image into the projector and where the image shines onto the wall, you can then follow the line and thus creating a larger line-art copy ready for colouring in.
Of course it is long winded and not everyone owns an overhead projector but if I have the means to do that, that's certainly what I would do. But once you can do that, then the possibilities are endless.
Actually, come to think of it, I think I might even "steal" that idea for myself.
Oh! Oh! Got another idea!
Get the chosen image to show up in landscape mode (to one side) on a large LCD TV (40 inches is bigger than A2 paper). Tape or blu tack the A2 paper to the topside frame of the telly (not on the screen itself!) and carefully trace the line art - don't press on the paper too hard what with the LCD screen behind, just very lightly follow the outlines using a soft tipped felt tip pen or a very soft pencil (eg: B7).
And viola, you have an A2 size line art copy ready for colouring in.
