Paper sizes large

can i use A0 Sheets with sketchup
can i choose the size of paper in layout

You can create a template for any size paper you want in LayOut

As for paper sizes in SketchUp, it depends upon your default printer. If your printer has A0 paper in it, you can select that paper size.

Or if you haven’t actually got an A0 printer to print to, you could print to PDF and (depending on how your computer does that) you may be able to select an A0 paper size.

Or you could install a driver for a ‘real’ A0 printer, even if you haven’t got one, then print to that but choose PDF output.