For A3 you'll really need to increase the resolution of the skin - 2048 minimum, 4096 best. Do that and I'll have a go when I get round to it - is there a time limit?
Yes, don't just resize it, that'll be exactly the same as it being 1024. The way to do it it to make it from scratch. Use Vectors too if you can, as they are scaleable, and hence can be resized with ease.
yep, u could always use Adobe Illustrator or Macromedia Fireworks or Freehand to make ur skins in vector
none of them is as easy, and as powerfull as Photoshop though