You'll need to check with your image software, printing resolutions are expressed as dpi at the print size. Images from digital cameras usually default to 75 or 150dpi at a rather immense physical size. Scaling the image to half the size will double it's resolution.
With that covered, here is a image resolution guide for printing:
Large format inkjet posters = 150dpi for A2 or larger lowering the dpi is ok as the posters get larger because you are generally standing further from the finished product when viewing, eg 60-75dpi is good for A0.
Smaller format laser/inkjet = You can get away with as low as 150dpi if you don't have 20/20 vision or plan on looking at it very closely. I would recommend for a quality result using 300dpi images.
For commercial offset printing = 200-300dpi.
I hope this helps!
Rob