I'm not an expert, but I already did this once for a 5*2.5 meter tall banner so here's my 2 cents :
1 ) customized sizes : in the camera panel, you can enter any pixel ratio you like.
If you look in photoshop, you can easily make a photo in correct size and see how many pixels it has, and enter that in LW.
2) a problem is that LW renders in 72px/inch and I don't know a way to change that. So if the printer works at 300px, I would up the size of the pic maybe 4 times and then resize it in photoshop.
3) absolutely no idea
