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The ISO A series is built so each size is half the previous one with the same 1:sqrt2 aspect ratio, starting from A0 at one square metre. A4 is 210 by 297 mm, which at 300 dpi is 2480 by 3508 pixels - the standard print-ready canvas size. US Letter breaks the pattern entirely at 215.9 by 279.4 mm.
A series
each size halves the previous, aspect ratio 1:sqrt(2), A0 = 1 m2
Pixel dimensions
pixels = size in inches x dpi
Because it is the only ratio preserved when a sheet is halved, so scaling between A sizes never changes the proportions.
300 dpi for photographic quality, 150 dpi for large posters viewed at distance, and 600 dpi or more for fine line art.