Keep in mind you don't need 300ppi for large print like billboards/posters etc.
Keep in mind that you could possibly create in vector and have no resolution issues at all.
A bit more about your project would be helpful to direct you further.
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Nah - your file would be unworkable due to it's massive size.
What's the content of the billboard? Have text? Solid background? Have text? Illustration?
Photoshop may not be the best program - InDesign or Illustrator could be better suited and you don't need to worry about resolution if it's all Vector artwork.
You could set it up at 6mm x 5mm and have a tiny file size.
And then that can be blown up to 6m x 5m easily
Most importantly - speak with your print provider - they will tell you what they need exactly
You don't need 300ppi for large billboards/wallscapes
30ppi would be fine - but depends on viewing distance.
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Instead of 6m x 5m
Set up at 1/10th scale
600mm x 500mm
at 300 ppi
When blown up it will be 30ppi
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Take the same thing and do it at
Set up at 1/5th scale
1200mm x 1000mm
300ppi
Blow up by 5 times - and it's 6m x 5m
When blown up it will be 60ppi
