Hi zoossh,
My experience with doing a poster last time is i work direct from a canon RAW file in photoshop(6MP size), then i work all the graphics and text from there. After which when i send for printing(printed a few copies), I tried two methods of enlargement.
For the first print, i went to Image>Image Size, and enlarged the size to A0 size, i checked the boxes Scale Styles, Constraint Proportions, and Resample Image : Bicubic. I left the DPI at 300
For the next print i did the same, but i did not enlarge it all the way immediately, i scaled it up about 20% each time until i reached A0 size. DPI 300
For the last print, I scaled it directly to A0 but i DID not check the Resample Image, after enlarging the size. This causes photoshop to drop the DPI accordingly, to about 100+ (can't remember the exact figure.
From the 3 different prints at A0 i find that the last method works best as it seems more sharp to me. Anyway i doubt there are printers which can print at 300 dpi at that size anyway, at least not in most shops.
I'd refrain from working at A0 size directly as it'll really slow down the performance of your computer. All these is assuming you work only from photoshop, and not from illustrator or freehand.(i work from photoshop most of the time too, more convenient to edit photos and design at the same time, not the best way, but my personal preference. :bsmilie: )
Just my 2 cents.
HTH.
