Photoshop :: Correct Way To Setup Artwork For A Billboard?

I need to set up photoshop artwork (which consists of about 5 layers) for a 6m x 3m billboard. I set up my whole billboard in Illustrator (at 3m x 1,5m - half of actual size) and placed the psd image in. I set up my photoshop artwork at a third of actual size (3m x 1m at 300dpi). However, this made my file VERY slow to work with (result was about 700mb!!!!). Someone suggested that I downscale the image to 500mm wide and choose resample image > Bicubic Sharper (best for reduction). This scaled my psd file to about 32mb, which worked much faster. Then I saved the psd file as a Tiff with LZW compression without layers and placed into my illustrator file. However, when I export for print as PDFX1a- 2001 my file size was only 17mb!!!! Did I do the whole process correctly?! Will the print quality still be fine for such a big billboard? Is there a better / standard way to set up photoshop graphics for a billboard?