Basically you can't change the scale of everything in one step. What version of SketchUp and LayOut are you using? Your profile says you are using SketchUp 6 but I expect that's not right. As you probably know you can change the scale of the viewports via the Scale setting in the SketchUp Model panel. If you have things you've drawn in LayOut using the Scaled Drawing option (assuming you are using a version of LO with the Scaled Drawing feature) you should be able to select those things and change their scale in the Scaled Drawing panel. Font size will likely need to be adjusted in the text settings and, of course, you'll probably need to move dimensions, labels and other text to get them positioned correctly.

If you need to change the paper size for a project, you can do that in File>Document Setup>Paper but you still need make adjustments in the size and placement of the content.

I think it's a good idea to use Layers in LayOut to separate content by type. All SketchUp viewports on their own layer(s), dimensions on another layer, text on its own layers. That makes it easier to manage those things because you can lock or turn off layers for things you don't need to see as you are working.

FWIW, if you need to do this sort of thing in the future, it's better to start with the larger paper because the exported PDF can be printed on smaller paper if needed.