When you draw a guitar shape in autocad, you need to make it right dimensions, you need to know dimensions of your bridge, neck and stuff. If you didn't do that, you just scale it to make it right dimensions. W
When you want to print it 1:1, just select that option in print menu (that is if you have big printer that can accept A0 size paper. If not so, you can draw 4 rectangles across the body shape, so it fits in them four. When you print, select 1:1 option, and take ''window'' command, and then select first rectangle, print it, select second one and do all four.
Dimensions of rectangles should be 210*297mm (if your printer is normal, A4 size)
I hope I've helped someone, and sorry for my not so good English.