what should be images size to fit A4 Paper?

and i want to make its width and height same as A4 Page

can ay one tell me whats the 开发者_StackOverflowsize?

note : not neccessary to be the exact width and height !!

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It depends on the resolution:

72 dpi (web) = 595 X 842 pixels 
300 dpi (print) = 2480 X 3508 pixels 
600 dpi (print) = 4960 X 7016 pixels

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As can be seen here (wikipedia) you will need to make it 210mm × 297mm.

Converting millimeters and centimeters to pixels (and the other way around) depends on the DPI of the screen you are using - it is not a constant.

See this SO question and answers.

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My local photo lab told me A4 dimensions are 21x29.7cm and 8.3 x 11.7 inches.

But, and this is just FYI, when it comes out of their machine you do lose a couple of millimeters from each edge, so most important is to make sure you have configured the output from within your postprocess software (i.e LightRoom or PhotoShop etc) otherwise the composition will be screwed.